We have Ooma and love it!
We used to have Vonage (and we loved them too) but swtiched to Ooma since there is no monthly bill. The switch was super easy! We got to keep our phone numbers too. The only difference I have found so far is the voicemail and using 3-way calling. There isn't anything wrong with the way they do it, it's just a little different from Vonage so it takes a minute to get used to. I really don't have any complaints!
I would definitely recommend Ooma!!