I'm currently testing out text+ for free texting and fongo for free calling. Both provide unique telephone numbers and can text or call (respectively) landlines and cell phones in Canada and the US.
The call quality of most of these apps will actually be better because its something called 'wide-band'. Which means its a wider frequency band, so you more bass and high end.
Some cell phone support wide band over GDSM but most cell phone companies don't YET. But they will eventually.
Where the VoIP over iPhone apps can suffer is reliability (more dropped calls) and delay. Both of which will always be worse on a slower connections. My most recent tests have been satisfactory and I am considering dropping my cell plan altogether. That's why I signed up. I need someone to take my sweet Rogers accnt.