Bivvies work great when the weather is mild or when you're moving sites a lot. You don't want to be "tent bound" in a bivy or it gets old REAL quick.
Google for "window film ground cover" or "Tyvek groundcover" to put under it. Make it a bit larger than just the bivvy so you can stash extra gear on it. Helps keep the bivvy cleaner, drier and minimizes wear and tear.
I actually have a bivvy I made out of Tyvek Homewrap. It works, but you have to wash it in a machine a lot before it quiets down. =)