I've have Vonage for about 4 years now. I have, on a few occasions, needed to reboot their box. Other times, calls have had issues that were corrected by hanging up and re-calling.
However, 99% of the time, it's worked without issue, and the plethora of features is amazing for the price. Plus, you can bring your number with you anywhere with a broadband connection. For me? It's been a very good decision to switch to them, but YMMV. Obviously, it requires a broadband connection to work.