We have had it for about a year now and love it. Originally, we used it with a couple of regular phones and split the lines (which meant running phone lines everywhere), but now we have one of those phone systems where you just plug the extra receivers into an outlet (not a jack) in the wall.
Really can't beat a $30/month phone bill with unlimited long distance.
The thing that took the most getting used to was having to dial the area code when just calling down the street, but you get the hang of it eventually.
As to the 911 service, I'm still trying to figure out what happened in Florida - Vonage makes it pretty clear that you need to set up the 911 service - it's in the manual and online in big red letters. We did that when we initially set up the service, and have had to call 911 a couple of times and never had an issue with it. Maybe that varies depending on where you live?
Circuit City occasionally has the box on a "free after rebate" deal - we got the box for free, and just set up the service and were good to go.