I posted a detailed analysis of wi-fi call latency on my +Republic Wireless phone. I quantified the lag by recording both ends of a call from my landline and measuring the time between me speaking and when the sound came from the handset speaker. I typically heard between 300 and 450 ms of delay. Interestingly, it's asymmetrical with landline-to-cell latency consistently 150 ms higher.

Another VERY interesting result which I didn't discuss much in the article is that calling from cell to landline gets about the same latency as calling from landline to cell ... as long as you stay on wi-fi. But if you handover to the cell network, then calling cell to landline drops the latency to near zero but the reverse keeps the higher latency. This result may be due to Republic's implementation that actually uses two phone numbers for incoming calls to the cell phone and routes voice between them, while outgoing calls just use the quasi-secret second number and not your primary number.
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