I have had this phone for two weeks now, and in that time I've been bowled over by it's speed, stability and all round performance. Previously I had an iPhone and I can honestly say I don't miss it at all.
I feel Microsoft have done a great job with the OS, it's very intuitive to use and looks fantastic, and actually makes both iOS and Android look extremely dated. The app situation isn't quite as bleak as some say, in fact there are some excellent free ones available, and I've found decent equivalents to all my iPhone apps.
I've read several reviews on this phone and they keep mentioning the 'low' screen resolution - this really is a non-issue, the display is outstanding, and colours are nice and vibrant too. It's great for showing your photos (incidentally albums are synched with Facebook which is another positive about WP7).
Internet browsing on this phone is a superb experience, the IE9 browser works nicely and is very quick on a 3G or wireless connection. It doesn't support Flash though which is a bit of a shame.
As far as I'm concerned the only downside compared to the Apple is that the WP7 music player doesn't support gapless playback, but in the scheme of things this isn't a major issue. The sound quality is excellent.
All in all an excellent phone. I have no regrets. It's more unique than an iPhone (or a boring Android phone), and I know it's subjective, but I think it's better.