I have the HD7 - Post Mango (it got the upgrade 2 days after I bought it).

Multi-tasking - not so much. Maybe when more apps are written to handle WP7's version of multi-sleeping it will be better. For now, 3rd party app restart on waking up.

The phone is fast, the UI smooth. It has some good integration of fb/twitter status info (though split between People and Me). No direct launch into the appropriate app from there, however.

If people don't like the grid of icons for applications on the N9, they're hardly going to love the single column on WP7!

Live tiles, when more apps make use of them will be good - Weather.com does a nice job of that. Being able to "pin" m.iCaltrain.com search results page for train times directly to the "Home" screen was also good.

There are swipe-like gestures in WP7 - but there are still buttons "off the screen to cause actions on the screen" - which goes against the design aesthetic of the Swipe experience. And I can't see MS letting go of the Windows button, can you?

I find the interface a little 'scattered' - even with the effort to pull things together into hubs.

There are things I miss about my N900 (h/w keyboard for one) - currently relegated to a drawer. The HD7 will follow it when my N9 turns up I'll cope with the absence of a h/w keyboard