My husband and I switched to Vonage in September. If you want to transfer your number, be very careful! We had a HORRIBLE experience getting our number transferred. Eventually, I go so fed up that I contacted the corporate "compliance" line and filed an official complaint of fraud and customer mistreatment. Because their compliance line is answered by a third party, I knew that they would stay on top of things. By 8:30am the next morning, my phone number was successfully transferred to Vonage (the problem was on Vonage's end) and I was contacted directly by someone from the corporate office.
Since then, we haven't had any problems with Vonage and it's super cheap (like the other posts said). It's great to be able to check your voicemail messages online, especially when you're on vacation. Just know that if you have any problems with the service, their customer service STINKS. The front-line answer center is in another country and they all have very thick accents and clearly read from a script. If they eventually transfer you up the ranks, you'll get a department that's in the US and someone who speaks perfect English and doesn't understand why it took the call center so long to transfer you.
FYI - if you ever decide to change from Vonage (or any other VoIP) to someone else, you'll leave your number for good. Once Vonage takes it, it's theirs and you can't take it to another provider.