I just cancelled my service with Vonage. We've been pretty happy customers for about a year. However, I am subscribed to their 500 min/mo service plan, and the rates went up from $15 to $18/mo. Also, because I'm getting married (and seemed to be glued to the phone all the time now), I've been going over my minutes, so my phone cost $90 total for the last two months. They charge 1.5/minute for minutes over 500.
The only reason I'm switching service is because I found a better deal. For almost the same price as the 500/mo plan with Vonage, I can get unlimited calls through ITPvoip.com. Although their 500/mo plan was very attractively priced ($10/mo), I went with unlimited because their overage charge is $2.5/minute, which would rack up fast. Plus, I used a link to their site from VOIPreview.org, and it gave me two free months of service and a couple free features that normally are pay services.
The biggest pro we have enjoyed with Vonage is the router they offered. It is very high quality, and we've always had a good signal quality from it. Second biggest - getting our voicemails via email. However, the voicemail feature is also offered through the new service.
The biggest con we've found was that sometimes during peak daytime hours, the phone will get tinny. I can't really blame Vonage for this - it's our broadband provider that annoys me. However, it's a definite downside to voip service, and I happened to have Vonage when it occurred.
I don't remember every having to deal with customer service, so I'm guessing I'm lucky. We never ported a number or had any need for any bells or whistles, so the base services we used were excellent. I would recommend Vonage (if it were cheaper!) to more technically inclined people - my fiance and I are both IT techs, so I think that helps with our positive experience with voip in general.
I really enjoyed the Vonage service, and I felt that they had a great array of services offered with the phone. I hope we have an equally enjoyable experience with itpvoip, but without the cost.