I usually carry an orange bag, especially in the hills, but would not use it as a bivi bag its primary purpose is to act as a signal if you get into trouble.
On a recent first aid course we discussed their usefulness with mountain rescue personnell, as there are bothy bags and bivi bags that do the job lots better and the concensus was if someone sees the orange bag they will usually call out rescue but if they see a bothy bag it is assumed a group is having a break and ignore it, bivi bags just dont get seen!
The British army bags are excellent for the money, but if I find a decent priced Dutch bag my british one is going to get passed on to my daughter!
PS make sure it is the brownish gore tex british one, there was a nasty green non breathable one issued with arctic bags for a while!