During the brief period in which I used Google Voice on my Samsung Epic 4G, I noticed poor audio quality.
As far as I know, it shouldn't really be any different, as Google isn't brokering the actual audio... they're just connecting you using your existing network. I've certainly never noticed any difference on Verizon.
Not anymore. You can do VOIP calls through a browser, and I do them with my iPhone using the Talkatone app (free). (Ironic that this is through GV but Google's GV app doesn't support VOIP calls; someone else had to implement that.)