I agree with everyone's comments about the British Army bag.
I have the BA goretex bag and it's pretty bombproof. I'm 6'1 and there's plenty of room for me. Enough fabric to pull over your head and a small pack in foul weather too.
I tried someone's hunka standard, for size (I didn't sleep out in it) and it was way too snug. I like the fact that it packs down small into its own sack, and it's v light, but I'd be worried about sharp rocks etc with the Alpkit bag. The material just didn't feel that tough.
With the BA bag you can sleep on broken glass, nails, railings, flint spearheads, hot coals (sorry, might be getting carried away!) But you get the idea.
The army bag is much heavier, but for peace of mind, longevity and vfm, it wins for me.
Also I replaced the issue drawstring with thin paracord so it cinches up a bit smoother.