I have it, works fine for what I need (LOTS of calls to Canada and US long distance, I abuse it).
As far as their 911 stuff goes, you set it up for your location, I haven't used it so I can't really say how it works, but it's supposed to be the same as a normal phone.
I got an expandable set of 5.8 phones from Radioshack on a deal they were running, the main one with an answering machine and two extras for about $120, speakerphones in em and everything, been using them since last October and they have great distance and great clarity, very happy with them.
If my internet goes down, Vonage automatically forwards calls to my cell. You set it up ahead of time, so if your connection is messed, or power goes out or whatever, after so many seconds of rining (mine is set to 24, about 6 rings) it'll forward the call to your backup. That way I never miss a call, my Sprint phone will pick it up.