I know I'll regret this... but

I've shared my experiences with WP7 in and out of many threads.

It's "decent". A lot of room for improvement, Mango answers some of the concerns, but not all. Lack of apps that I've used for ages on iOS or Android is sorta bothersome.

UI is quick to maneuver through, ease of use and workflow are (for the most part) in WP7's favor. Lack of Flash in the browser is a problem for me - I'm an Adobe Flash/Flex dev. There are quite a few problems in some apps, for instance when you use Shazam, if you click on the YouTube link, it doesn't pick up if you have either the Microsoft YouTube app or the HTC YouTube app... still opens up in a browser and stalls half of the time.

Lack of true multitasking, not all live tiles are responsive or update properly, applications like IM+, MS Onit, and even Mosaic by Tribune are very likely to stall and freeze the entire phone. Camera is damn near instant - just hold the camera button from even a locked screen. For a 1ghz cpu, I can tap it out rather easily.

Development is another can of worms... but you asked my experience. Good phone, full of some annoyances, can be improved, connectivity to Zune (wireless sync especially) is great. Games/XBL implementation is awesome (I actually have an XBOX 360) but the fact that games that were free on Android cost $2.99 or higher is annoying.

I like it, but Android fits my workflow tighter.