In reply to Jon E:
Use a Mammut Granite 40l for Winter and Summer. Can fit everything I need in it and is comfortable to carry heavy loads with.
I've not had any issues with it whilst wearing a helmet. Sinches down enough for my needs but other sacs may do so more.
How it's going to fare after 5+ years of use I can't tell you yet but it seems pretty robust.
I'd prefer it to have a proper snowseal on the top but use a drysac in Winter anyway so not really an issue. The bungee ice axe attatchments can be a bit footerie so worth having a play with first.
If you want a simple, lightweight sac that carries well then worth a look. Took me a while to find a 40l one but glad that I did.