Just wanted to share my experience with Callcentric.
I haven't tried many VOIP providers: Broadvoice, Gizmo, Skype, Voipstunt and now Callcentric.
Recently had problems with my asterisk loosing connection several times a day. As it turns out, it was my ISP's problem but Callcentric's support people were incredible. They monitored my line for over two weeks, providing constant feedback. If I was paying a high volume plan I would expect that service level, but not for a pay as you go plan as I have. I would say their service is outstanding.
I hope this doesn't constitute shameless promotion, but I like to support companies doing things right, specially in the difficult VOIP market.