In reply to ibexguiding:
I have a 37+ (which is a bit bigger than the Arc FL45L which is 33L). Other recent sacs I have to compare against are an Ascentionist and a Mutant*. I have had none of the issues mentioned above.
Pros:
Properly lightweight
Carry's really well (surprisingly comfortable with a heavy load) - Trudging around in Scotland in Winter with a full pack plus skis hunting for snow was fine.
The back panel is quite stiff - Easy to pack as it sits straight and I don't to pack as carefully as the Ascentionist (quite handy in winter to just stuff kit in after topping out in bad weather).
Carry's a rope really well (for a no lid sac) - Contrary to a previous poster, I've found it carries a rope really well. I had the rope slipping off issue with the Ascentionist but not with the Tupliak. The Mutant carried a rope well but was a lot harder to overstuff.
The "grappler" hook is really easy, great in winter with gloves.
Unsure:
Roll top - The pack fabric is very robust and waterproof meaning things stay dry in the sac but the roll top for me is at times genius and others a pain - The jury is out.
Cons:
Lid Pocket - The idea is great for the floating lid pocket but it sometimes gets a bit stuck and inaccessible depending on how you pack the sack. A lot easier to get into than the FL front pocket though when packed from my one outing with a borrowed one.
Overall, it's my favourite all round sac I've had. Comfy to carry, feels out of the way to climb or ski in, robust, waterproof and simple.