I've been running the Developer Beta 2 preview on my WP7 for a few days now, and I can safely say that it is a massive step forward for Microsoft. There's a ton of stuff out there on the internet, but just some of the features that I absolutely love:
Quick Calendar Entry WP7 boasts one of the best mobile calendar apps out there (infinitely better than the crappy Calendar app on the iPad, for which I have a complaints list as long as my arm for), and they made it even better, adding support for multiple synced calendars (Windows Live / Exchange only for now...) and creating calendar entries even faster! Now when you tap on a timeslot in the day view, you can start entering the title immediately, and it defaults to an hour block. It's fast, good looking, and easy.
Messaging Threads Conversing via txt to MSN to txt to Facebook is amazing, and is really one of the killer features in WP7, as far as I'm concerned. A nice little touch is the automatic emoticons that come up now, even for text messages!
Battery Saver They're not kidding when they say it saves battery. WP7 now gives you a rough time-based battery life estimate in terms of hours (which is useful enough on its own), but also now has a battery saver mode which you can enable manually, or set to turn on when your phone is running out of juice. It pumped the life of my phone from 5 hours to roughly 10 - which is awesome if you know you need to receive an important call, but you're running low on power.
Chinese Keyboard Drawing Input Now this is pretty awesome. I don't know if anyone's detailed exactly how it works, but it seems accurate enough - you just draw your chinese characters straight onto the phone (of which I know very few, but enough to play around with this).
It's definitely worth the rather long update process to get your phone to run the Mango Beta - you won't regret it!