My biggest complaint about WP7 was, that it is really hard to get some hands on time to be able to judge this mobile OS. Microsoft seems to be aware of this and is providing a WP7 emulator that runs in mobile browsers.

Not everything works (apps) and you have to deal with some scripted actions, but right now I can play a bit with WP7 on my S2 (;

Open with your mobile browser: http://aka.ms/wpdemo