Have used a BlackBerry as a main device for years. I had tried a Samsung Focus for awhile as a 2nd device to check out WP7 while using my BlackBerry 9780 as the main device. Got tired of using 2 phones, so got rid of both of those and went with the HTC Radar when it came out since I liked many things WP7 has that BB is lacking in. With that said, I can see myself possibly going back to 2 phones, with the BlackBerry as the 2nd device.
I think the virtual keyboard is good, but doesn't compare to not looking at the keys while typing and typing with near perfect accuracy like I can with the BlackBerry keyboard. The virtual keyboard is pretty easy to use and does good on the auto-correct, but if I end up having long IM sessions or have to type a lot, the virtual keyboard becomes a pain pretty quickly. If you use a keyboard a lot and liked a physical keyboard, this part is a big deal IMO. Not a fan of any side slider keyboards either. I can type just as good, if not better, on the virtual keyboard than I can on those.
The notification system in WP7 isn't that great once you get past the core programs that come with the phone. The tiles are great themselves, but no LED notifier and no 3rd party icons on the lock screen is not so great. I get tired of unlocking the phone to see if I got something from WhatsApp or other IM programs. Compared to the BlackBerry, it's not close in my opinion in the notifications department.
Other than those two things and missing some key apps, I think the WP7 platform is really good. Email is considered one of the main things on a BB, but I like the email look and feel way better on WP7 than the BB. Battery life on the Radar is really good and everything runs smooth. Browser is better than the current BB browsers in speed and rendering from what I've seen as well.
I never had any problems with my BB devices like some people seem to have with slowdowns and reboots, etc. so I don't have that negative viewpoint of their devices. They always worked great and do what they do very well.