Well, I've been using Skype for a while, and the quality is good, with the occasional pop and warble. Personally, I'd love to see something with a quality more comparible to POTS.
Gizmo is also pretty neat, but I feel the quality is actually a bit lower, but others may have a differing opinion.
Vonage, only problems ive had are sometimes when I have voicemail and delete it my phone still thinks I have one. Not sure if thats a Vonage issue or my phone being a POS.
I've been using VoicePulse for almost a year and have been very happy with them. I use it as my main phone and do not even have a POTS phone connected. The only problems I've ever had with the phone service were due to interruptions from the ISP and not the phone service. It seems Cox internet loves to tinker with the service here. Then, that's the downside with any broadband phone; they all require a good signal from the ISP.
Only requires a Sipura ATA adapter and use any regular phone connected to it. I went with this company because it had pretty good reviews.