A word from a user of the iPhone 4 on Three...
Got rid of my iPhone 3G (well, it's still under the coffee table, but will go to O2 Recycle...) and managed to get one from a Three Store - eventually - about three weeks ago.
For £30 a month I'm getting unlimited texts, 1GB of data, more minutes than I know and unlimited 3-to-3 mins. The iPhone 4 16GB cost me £99 outright on a 24-month contract.
I've been really impressed by Three's network. Unlike O2, no more dodgy capacity and all connectivity dropping in and out in Lakeside, Bluewater etc.
At worst, it drops onto Orange's 2G network which is rock solid - or at least it's a good enough back-up (there was a big outage a few days ago - not that I noticed).
When they merge their networks with T-Mobile, presumably Three customers will get the best of 2G from T-Mobile and Orange and Three already site-share with T-Mobile for 3G sites - so a pretty good network to be on.
They don't offer any Wi-Fi bundled, but compared to O2 it's like being on WiFi more than 3G.
The only downside is their customer service. All out in India, all very much driven by hopeless scripts, all very difficult. Avoid if at all possible!