I've been using it to call Canada from the US for a few months and haven't ever had an issue. I probably call there about once a week, and talk for maybe an hour.
I've done it a few different ways too... Sometimes from my AT&T U-Verse Voice service, sometimes from my Sprint cell phone (by calling my Google Voice number, then entering the number I want to call.. basically using it like a calling card), and also from my Verizon Droid phone via the integrated Google Voice Dialing functionality. I haven't once had it where one party couldn't hear the other, or get a wrong number. I've barely had any incoming calls though, so I can't comment on that at all.