, I have used Groove IP, but there is a bit of delay, plus it doesn't handle network switching well (i.e. - WiFi to 3G, the call will drop). It would work well for a tablet experience, where you generally are not moving away from your connection all that often.
The major downside to it is Verizon. If I am on 3G and I receive a call over cellular, it will drop the VoIP call because Verizon can't handle calls and data at the same time. This makes it very difficult, if not impossible, to set it up reliably.
For example, I want my cell to ring if there is no data, but without data, I can't change the phones to ring. So, I would have to ring the cell and GTalk all the time. And, once the call comes in on the cell, it will drop the VoIP connection.
So, I just need Verizon to get there butt in gear on VoLTE, and all will be right with the world!!!