Awhile ago I tried Vonage for two weeks, and wound up returning it. Although its feature set is rich, call quality was rather poor. Very high latency made the call as bad as a cell phone call, with participants talking on top of eachother.
I recently tried Skype again (it had been years), and was blown away at the call quality; by far the closest thing to hard-line quality on the market; anyone suggesting otherwise thinks mobile phone quality (on even the best of carriers) is good. There was almost zero latency, which amazes me from a technology standpoint. I'm curious to know whether Skype is using TCP, or UDP.
Obviously Skype and Vonage are wildly different products, with wildly different feature sets, but, I passed my judgement based on the primary function they both purport to delivery, phone calls.