I have used GrooveIP for about a month or so, on Optimus V. I live in NYC, where I consistently get 1.5Mbps down, 600kbps up (using Speedtest.net bandwidth testing app).
I wanted to like GrooveIP, because it would have allowed me to stay on $25 plan. It is simple to configure and works as specified. However, there are two problems with it; one minor (which may have been fixed by the developer by now) and one bigger, which cannot be fixed by the developer.
The minor one related to answering calls to your GrooveIP (i.e. GoogleVoice) number. It often happened that, if I don't answer the call after first two rings, my phone switches to my regular phone dialer, the call is somehow redirected to my voice plan and, if I choose to take it, my voice minutes are used, as the call now goes over voice network, and not over VoIP using GV. I'm not really sure why or how this happened, and I wouldn't be surprised if this problem is gone by now.
The bigger problem was the reliability of voice calls over Virgin's (Sprint's) data network. Even with my pretty solid bandwidth, pretty much EVERY call I made over Virgins' network had a very long latency (3-10 seconds). By the time the other party hears what I said, I'm saying something else. Much like talking to Apollo 11 on the Moon. Oftentimes, the audio quality of the call was so poor (dropouts and digital distortion) that it was completely useless.
Bottom line is, while VoIP is conceptually an excellent solution for avoiding voice network and metered minutes, in real life, mobile data networks are simply nowhere near robust and reliable enough for acceptable call quality. The only time I had normal conversations through GrooveIP were when my phone was on WiFi (connected to a relatively fast pipe).
Eventually, I have upgraded to the $40 plan and uninstalled GrooveIP.
Your mileage may vary, though.