Wireless Info took the T-Mobile Hotspot@Home Wi-Fi + Cellular service into their own Batcave (similar to ours when we played with it) and subjected it to all sorts of tests. The result? Equally good sound quality on both the cell network and Wi-Fi, but a slightly glitchy handoff when going from Wi-Fi to cell (cell to Wi-Fi is fine). It's nothing to really worry about as long as you're not going in and out of range all the time, so if you're one of those poor dudes who can't get a signal to save your life, this could be the plan for you. [Wireless Info]
I've got the Nokia 6086 for this service and I'm using the newer Apple Airport Extreme for my wireless access point. It took a bit of effort to get everything working but the calls are very very clear. I'm using the phone for my long conference calls.
The only problem I'm having is that the Airport Extreme tends to reboot itself in the middle of a call. I've got the latest firmware updates but so far I'm still having issues. Apple hasn't been much help either. If it doesn't correct itself soon, the router is going to have to go back.