I'll give you my experience with Vonage, and that was that it was a very disappointing experience, which after months of terrible call quality and long support calls to try and resolve the problem, I ended up having to cancel, and then being charged to cancel, even though I was cancelling because the call quality was so poor (callers would say that I was breaking up, there were long delays on the calls etc).
I then moved to Voipfone, and everything worked perfectly - no more complaints from callers not being able to hear me.
I'm now on Gradwell (purely because the of the back office system) and I really rate VOIP - but Vonage nearly put me off it for good.