I'm going to make a slightly unusual argument that the filling of a jacket isn't that important, or at least only secondary. Most people favour synthetics over down because of their ability to remain warm when damp (icefalls can be dripping with water and steaming even when the air temperature everywhere else is -15), but both RAB and PHD make down jackets with both water resistant outers and -very importantly- inners and that should get around this issue to a great extent. A warm jacket is a warm jacket whether it is stuffed with down, primaloft or cheap no-name polyester. Of course weight and compressibility (and price!) will change with those, but once it's on this matters less than design and cut. What is really going to make the difference is "usability" for the want of a better word.
Getting dangerously cold in the mountains normally results from a series of bad judgements - a vicious circle. When you think "shall I put my belay jacket on? Nah... I can't be arsed with the faff" that's a bad thing. The next thing is you decide not to change your damp gloves for mitts despite having numb fingers, then it's not pulling your hood up and before you know it, its hypothermia. I remember clearly watching this happen to a mate in Scotland to the point where I was slapping him and shaking him to get him to do stuff. Faff is your enemy - any thing that makes the jacket faffy makes a it a bad belay jacket.
Last Easter I did some multi-pitch ice climbs in Lyngen, Arctic Norway, with two mates. The weather was clear but cold, -10 or below and gusty, stripping the warmth away from you rapidly. For belay jackets I had my now 8 year old Patagonia DAS Parka, my mate Simon had a big warm Marmot down duvet and my other mate Jody had the basic Alpkit down jacket. All of them are fine jackets in their own rights for different thing but it became so clear to me why the DAS is so much better designed as a serious belay jacket. From now on I won't buy another belay jacket if it doesn't 1) have a hood that easily goes over a helmet (and a shell jacket hood already over the helmet) and 2) if it doesn't close right up to cover all your face with a single zip (usable with bulky gloves on). It hadn't really struck me how important this is until I watched my mates struggle with their jackets. Simon's Marmot had a really big, protective hood, but being the detachable design it has some complicated velcro and poppers to close it up over the face. I noticed that this was a hassle and Simon wasn't able to do it easily with his gloves on and therefore wasn't, despite suffering from the cold. Simon goes against all his native Australia's stereotypes - being gently spoken and quiet. When we were together on the belay I was unsure whether his quietness was just his normal, polite nature or whether he was starting to get deeply cold and moving towards the first stages of hypothermia. At one point I actually did the hood of his jacket up for him and resorted to once in while slapping him on the top of his helmet to see if he seemed to care! Overall Simon was OK (except for the large chunks of ice I managed to hit him with later - sorry Simon!) but said he was really miserably cold on some of the longer belays. Jody's Alpkit duvet couldn't even be done up fully to the neck easily with his Montane Extreme smock underneath and the hood poppers that would cover the mouth couldn't be done up either. Jody hails from hardy Yorkshire stock so suffered manfully without moaning but also said later he had felt miserably cold whilst belaying. In contrast with the DAS I just pull the hood on and then zip it up easily over my softshell and fleece neck gaiter-thingy with out any problems at all and when fully done up little but your eyes is exposed. It was still a cold day and my toes and fingers felt it, but I could climb in a stretchy and relatively light softshell but my core remained warm putting on the DAS at each belay. We all survived and did the climbs in good style, but my jacket simply made the experience easier for me.
This isn't meant to be an advert for Patagonia, but the DAS Parka is everything a belay jacket should be and you can measure other possible contenders against it. It's not faffy at all. It is the "anti-faff". Even when wearing mitts your hands slip through sleeves; there aren't wrist adjusters to faff with; there is basically no hood adjustment - just pull it on and zip up. The less faff, the quicker it is to get on and start warming you back up. Hence I'd stay clear of any jacket with a detachable hood as they are always going to need some needlessly complicated hood doing-up system that is separate from the zip on the collar of the jacket and there is even the risk that the hood can get detached - a thing that happened to a mate's Rab Andes outside of the CIC after an epic descent many years ago. He fortunately managed to step on the hood before it blew away. Less fortunately he was still wearing his crampons at the time. The fact that a detachable hood sits outside the collar to the jacket means that that collar is going to be tighter than on a jacket with a fixed hood. This may seem snug over a t-shirt but could well mean you can't do up the jacket fully over the bulk of a hooded softshell or shell underneath - particularly if you have some type of scarf on. These seems to me to be a major problem with detachable hoods. Likewise any jacket without a hood is not a serious belay jacket. Classic, lightweight down duvets - I'm still happily using a North Face Nuptse that I bought in 1995 - tend not to have hoods, and whilst being great for lunch stops when ski touring or belaying at the crag on frosty mornings they are not the best for full-on mountain misery. The good belay jacket really needs a hood you can hide in - I have a North Face Redpoint Optimus which fits many of the other criteria of a belay jacket but it let down by its hood. On the one hand it is a simple, permanently attached design that is easy to do up over a helmet with just the zip, but on the other it leaves too much of your face exposed - not providing enough protection. It is fine for icefall cragging in quiet Finnish forests, but not so good on spindrift-blasted mountainsides. A belay jacket also of course needs to be big enough to slip easily on and off over whatever your normal climbing clothes are - particularly the arms have to be wide enough to let gauntlet gloves or mitts pass through smoothly. And because you will be wearing it over possibly snow-splattered or ice-hardened clothes, it needs to be water resistant on the inside as well.
This isn't a review of different belay jackets, as I've really only used the Patagonia and North Face ones mentioned above. I did recently try on in a shop the new Berghaus primaloft jacket - I think it must have been the "Combust". It is worth a mention because it had a huge fitted hood that would easily swallow the bulkiest of helmets with ease, and seemed to tick all the other boxes for simple, easy to use, anti-faff-ness as far as one can tell in a shop. Kirkpatrick has been a big proponent of the belay jacket concept for many years so it is perhaps no surprise now that he works with Berghaus that they are turning out top of the range technical gear that seems at least to make no compromises to anything besides serious mountain use; even its price isn't too bad. RAB make the helpfully named Belay Jacket, but it comes with a detachable hood. RAB work with some of the best climbers and designers out there so this to my mind is odd - I see no advantages to it in hardcore use and many disadvantages as outlined above. Of the RAB down-filled jackets the Batura or one of the Neutrinos all come with fixed hoods so might be worth trying. Mountain Equipment makes the Fitzroy - a name that suggest they get it, and Haglöfs make the Barrier Zone Hood: both jackets very much look the part, but again I haven't worn them. Try before you buy and try wearing what you would normally for climbing plus helmet and mitts. Many of the North American firms besides Patagonia make 'core looking belay parkas - but they are much harder to find over here. In Canada the Mountain Equipment Coop do some ridiculously well priced ones if you happen to be over that way. In the thread that prompted this essay, the Wild Things jacket came highly recommended, and was also said to be available from Peglers in Arundel so might be of interest to readers in southern England. If Peglers chooses to stock something that is a pretty good recommendation in itself. Finally, a possible joker in the pack, the Bionnassy Warm Jacket - Decathlon's own brand. Decathlon have a bit of tendency in their mountain clothing to look at what works elsewhere and then to ruthlessly copy it using non-branded but perfectly good materials. This must be really annoying to innovative companies, but here is a belay jacket that looks suspiciously good and only for fifty quid. I tried one on a year or so ago and it seemed well designed, but I'd be interested to hear from anyone who has actually used one in the mountains where niggles in the design quickly become huge issues. Other thoughts: There is a review of the Mammut Stratus belay jacket by Jack Geldard - UKC Mammut Review. Another possibility is the Marmot range: Marmot Website as they do have some designs that have substantial hoods with a single, easy, zip closure.
UKC User Morgan Woods made an interesting comment: "I am very happy with my Mountain Hardwear voodoo belay jacket (now replaced by the Compressor). Another factor you may like to consider is how quickly a jacket will dry. My synthetic one can usually dry out overnight in a decent dry-room but a down jacket would struggle."
Also, the Buffalo belay jackets are another - slightly different possibility. I used the original one for years with the full Buffalo system.
Having, say, a down jacket without a hood or with a detachable hood is still a hell of a lot better than nothing when your mate takes two hours to dig up the crux in a Cairngorm mid-winter maelstrom, so don't despair if that is what you already have, or if all you can afford is a (very reasonably priced) Alpkit Filo - just 'man up' and try to have fun! But if you are thinking of specifically buying a belay jacket, or are in the market for your first insulated jacket for climbing of any type - hopefully the thoughts above will help you sort through the contenders. I'd say see you up there, but you won't because I'll be the one hiding in the hood of the big DAS Parka.