I'd love to find some rigorous way to measure it, but (as of six months or so ago) when I sat with two phones, held down a button, and waited for a tone to come out the other end, the delay was observably longer with a GV-to-GV call than with a GV-to-cell call, which took longer than cell-to-cell.
I find myself saying "no, go ahead" or talking over the other person far more often with GV than not. It's certainly usable, but I avoid GV if the call quality really matters.
It definitely takes longer for calls to connect and I think it may drop calls more frequently when using it on a cell (although it has improved greatly in that regard from about a year ago). I recently started using it through gmail and have been told the calls sound much better. Not sure what it all means.
I find myself saying "no, go ahead" or talking over the other person far more often with GV than not. It's certainly usable, but I avoid GV if the call quality really matters.