In reply to Nick Smith - UKC:
I have 3 synthetic jackets, and I like them all.
Oldest is a Berghaus something. 20 years old, very sturdy, cosy, but not too light - about a kilo.
At the other extreme is my Montane Solo, nearly 10 years old now. This is much lighter, has no hood, and is not so hot (obviously). It is Primaloft filled and pertex outer, which seems to me to be the lightest for the warmth combination.
In between I have a MH Compressor jecket, with hood. The hood makes a big difference to warmth.
In general I find Prialoft jackets can be quite sweaty if I am active, unless the wind is very strong. So I wouldn't use any of these as a mid layer unless I was stationary or I was in a real hooly.
Mostly these jackets go over the top of my outer layer.
My outer layer of choice is the Paramo Velez light smock - brilliant bit of kit. Waterproof and as breathable as a jumper.
My layering system would be to wear base layer and close-fitting jumpers or fleeces under this according to warmth. Once we get below freezing, then the primaloft jackets come on as belay jackets, ie fling them on top of the Paramo. This works very well.