I dont like the IM options and the way it is handled at all. I guess mango will fix this, but the apps shut down for the most part when I go back to the home screen, so any messages sent, when I get an alert and go back to the app (say IM+ or something), it has to log me back in before I can reply. Thats annoying.
Of course most people I just text, but there is a group that I use IM services for. The ones I was using with android are not available with wp7 so far.
I am still uber impressed with the smoothness in which this phone/OS operates. It is similar to iOS in that regard.