Had a play with this in CPW yesterday.
In terms of hardware, the Lumia 800 looks and feels great. It's got this curved unibody, feels good in the hand and attractive. The CPW has a working model on display, so had a look at the OS; WP7. I was pretty impressed, though not sure if I'd want to go down this route for my next phone (6 months time). The tiles were quick to respond to the touch, the screen is high quality, scrolling on the internet is slick indeed. Would take some getting used to. No real lag that would cause a user to balk when navigating around. When these come out in their various colours, can see them being a hit with a younger demographic. Handset price probably needs to drop to about £300 on SIM free though.
However, drawbacks for me include the non-expandable memory, the Xbox branding for games and the reliance on Zune for music synching. I've read that you must use Zune Player to synch mp3s, whereas I tend to use WMP for transfers / drag n drop.
I don't want an iTunes type store / interface messing with my music library but if anyone who uses a WP7 can advise on this, happy to hear.