I have been using it for a long time from Sprint. Works fine, no difference in quality if it is over the cell or wifi. The only issue that I had, but it might be how Sprint implemented it, is that it won't switch a call from one to the other on the fly. If i start a call in my house over the wifi and then i leave the house, it will drop the call when I get out of range instead of switching over to the cell.
I have the same issue as you, poor signal in the house, and some of the calls go directly to voice mail. But wifi calling took care of that.