Good article here from O’Reilly: Top Ten Questions People Ask About Switching to Internet Telephones

Very informative and generally very accurate based on what I know from my experience. Also from my experience:

Stay away from Vonage. They’re terrible. Their marketing materials say transferring your local number to Vonage will take a minimum of 20 days and shouldn’t take longer than 30 days. I finally cancelled my Vonage account after waiting FOUR MONTHS for my local number to be transferred. I spent hours and hours on the phone with their completely unhelpful tech support before finally throwing in the towel and cancelling. I’ve now switched to Broadvox, which seems to have infinitely better customer service. Our friends Vince & Staci switched to Broadvox several months ago and have been very happy with it. And you can’t beat the rates - $9.95 per month for unlimited incoming calls and 100 outgoing minutes, and only 2.9 cents per minute for outgoing calls after that (or $29.95 for the unlimited plan).

I’ll say it again: Vonage is terrible. Don’t use Vonage.