I’ve had Vonage for over 2 years and with the service I’ve had from them I would never consider any other VoIP carrier. I know there are some slightly cheaper, but Vonage has been more reliable than my old land line from Verizon.
I’ve had one problem, it turned out to be my Vonage router, which they replaced for free even though it was over a year old.
And yes, you get to keep your existing number.
EDIT:
A warning though, VoIP is only as good as your internet connection. I happen to live in an area where Adelphia is rock solid. If your high speed is less than 100% reliable, your VoIP will be the same.