In reply to Onyx:
Join the TA- get one for free!
You can buy them from quite a few places, probably try Silvermans as a first point of call (though they are expensive):
http://www.silvermans.co.uk/
They are not too bad, in truth it is rare that you will ever need anything else, though they are not mega breathable, as I don't think they are true goretex.
For alpine use I use something that looks the same, but is made of a lighter pertex (i.e. not waterproof material) material, that is dirt cheap and I don't care if it gets ruined.
However the use of this type of bag depends on what you intend to use it for.
I think if you are intending to go out for anything longer than one or two nights in weather, take a tent, a lightweight one is worth the weight, and if the weather is so bad that you need anything other than a jetpacker then you're looking at a real mountain tent. And the irony is that if you're needing a real mountain tent, then it's snowing and you therefore don't need a totally waterproof bivi bag.
Remember that the military intend them to be used with a 'basha' to give some overhead cover, and comfort is a very low military requirement. They also have really warm sleeping bags to go inside.
So- as always- you have to think about exactly what you're going to be using this bag for.