That's a really good question.
Although I've never played with VOIP, I have been watching from the sidelines.
I have noticed that a typical cell-phone conversation has noticeable latency. It's bad enough so that, when I'm calling from my cell phone, I try to call a land-line if possible. I don't want to incur the full latency of a cell-to-cell call.
I have often wondered what the source of the latency is. Does one handset have worse latency than another because of variable processing power? Or is the latency dominated by delays in processing at the tower? Or somewhere else? Is the latency asymmetric? For example, is it mostly on the encoding side in the handset and not so much on the decoding side? It would be cool if an expert would drop by this thread to enlighten me.
Anyway, if latency were a real issue for VOIP (and I have no experience to say one way or the other), then, even if the audio fidelity of what comes through were substantially greater than that of a land line, I'd still prefer a land line for a real-time conversation.