Much as I hate Apple's "We are God" attitude, some of their kit is very, very cool.
The iPhone has by far the best MP3 player interface I've seen. I've tried a Window's Mobile 5 device, and the integration between that and Media Player is rubbish (It switched off the sleep-mode, but doesn't dim the screen automatically, for instance), and my w910's interface was just - well - nothing like as slick.
The iPhone touch screen really does blow everything else away.
One warning though - the headphone socket is *not* a standard headphone socket. If you don't want to use Apple's crappy white things (iPhone or iPod, weirdly enough, both work), you need to buy a third-party adapter cable. This is different from the iPod Touch, where you can use any headphones.
Also (that makes two warnings) - no bluetooth functionality *apart* from the headset profile. So no sync-ing with your Mac/PC unless you use the cradle and iTunes. That includes appointments, contact addresses/numbers etc. And no bluetooth copying from/to other devices at all.
On the plus side, you can unlock it and run it with any provider. Just be careful if you've got no inclusive data on your contract. I've put garbage in as the ADTD dialer string, so it never tries to use data unless WiFi is available.