I have been very impressed so far with Obivoice - both in call quality and responsiveness to users. A number of improvements have been imlpemented, including a couple I suggested. They monitor the forums and respond to their own "request a feature" page.
Granted, they aren't as "matured" as other offerings, but it's kind of fun when your suggestions are implemented, and you get an e-mail saying so.
$60 for 2 years well spent, and I've only had their service for a couple of weeks.
Time will tell if they can keep this pace up (and keep the price down), but they are off to a great start!