I've been a nettalk user for about 2 years and just got my obitalk today. I have the nettalk tk6000 which isn't available any more (it only required a one-time payment), but I strongly suspect that it's the same as the nettalk duo in terms of sound quality. The duo requires an annual payment. Nettalk doesn't offer the tk6000 any more.
Anyway, I started looking for a replacement device due to the static and choppy sounds I've been experiencing lately with the tk6000. Until recently it had, overall, been a great device. They put out a firmware update which I just received today, and that supposedly fixes some call quality issues, but I haven't checked it yet.
I just finished my first call with the obi100, and I have to say I am amazed with how clear the call is. It sounded almost like a landline. I've been using voip services since around 2004, which was when I cancelled my landline, and I've had several voip providers since then. Nettalk was about in the middle of the line in terms of call quality. From the *one* (lol) call I've made with obitalk, I'd put it at the top along with Broadvoice.
I was hesitant as I thought maybe the amazon reviews were paid for, but now I'm thinking maybe they really are genuine.
Of course I can't say whether your obitalk quality will be better than your nettalk connection, but for me there was a big difference between the two. I'll still use my nettalk as a backup line.
Good luck, and let us know what you decide and how it works out.