They're usually in pretty good nick on account of being indestructible. One thing worth noting, though, is that they're designed for soldiers wishing to keep a low profile. This means a) no hoop, and b) green.
Regarding a), if you get rained on, there will be puddles on top of you. Snow piles up too: it just doesn't have the half-cylinder shape you get with a hooped bivvi. Water on top of you is cold, and the bag isn't breathable if water or snow is covering its surface. Squaddies seem to just plonk it on the ground and climb inside whatever the weather. However, I have really struggled with them, when used without a tent, tarpaulin fly sheet, or some kind of natural shelter.
Regarding b), in an emergency, you will be less visible to MRTs, Sea Kings etc. You also won't spoil people's view quite as much as you would if you were day-glow. Make of that what you will.
Overall, £20-30 is a fantastic price for a bivvi bag. (It's also by far the cheapest way to obtain Goretex for making stuff like ponchos, when you look at the £/m^2.) However, think about whether the lack of a semi-circular hoop will be inconvenient for you, wherever you might be going, given the level of other shelter available.