WiFi on this phone has proven very beneficial for me recently. I first tried it when I wanted to send a bunch of pictures I'd taken on the phone to a friend. I put them all in one email (about 6 pictures), and the email just sat in the outbox and eventual presented an error message. On a hunch, I connected to Wifi, and was able to use that bandwidth for the email to go out. The phone 'seemed' to figure out how to use what services, and I seemed to get the benefit of both.
Then I went to Canada for a weekend. Not wanting to incur any charges for calls or data on my phone while in Canada, I put my phone into Airplane mode. Then, I tried turning on Wifi, and vioala! My phone was acting like a mini wireless computer. When connecting to wireless networks in canada, I was able to seemingly do everything but place phone calls. Emails and Web browsing were quite fast and easy. This was nice and easy to do, and very satisfying (except, perhaps for when I used it to learn that the wait at the border back into the US was 90 minutes!). Fortunately, when we were at the border, the wait was less than that, and I was able to receive my US carrier service again! Kudos on Wifi Pre!