I've tried Talkatone, Sipdroid, and Groove IP. GrooveIP is easily the best (sadly). Tighter integration sure would be swell, but it seems a lost cause. It works well enough, especially for receiving calls while on Wifi. Beats the hell out of paying for minutes, though.
I've been on that t-mobile plan for a few months using GrooveIP for GVoice-VOIP, but keep thinking there is a better way.