My sister has VOIP. She has a different service, though, that doesn't use a regular land line at all.
The company she uses does something that registers her address with 911 so that if they ever call 911 from their number, they know where to go.
The service she has is generally reliable. There is a spot in her house where she can't use her phone at all. And sometimes we will get disconnect or there will be a lot of crackling on her end. I'd say it works completely fine about 85-90% of the time. One quirky thing that it does is that it sends her calls to Privacy Director some of the time. Most of the time she calls, her name and number show up on Caller ID. But then some of the time she has to announce her name to Privacy Director, then it calls me and I have to press 1 to accept the call. It's very odd. She'll call and have it act fine, and then she'll think of something else to tell me right after we hang up, and it goes to Privacy Director.
I could get the same service as my sister and save quite a bit of money, but I haven't yet because hers is a little quirky. We may end up doing it, but we're planning on moving in a few months, so we'll probably wait until then. I'll definitely make sure that our cells get a good signal at our new house before we switch. VOIP is less reliable than landlines, so I wouldn't want that to be my only option.