I hate Microsoft... I truly despise them. I think they're an evil empire, and I try to have as little to do with them as possible.
Google has pretty much crossed over into the evil empire category for me as well. They aren't as bad as Microsoft, but they're bad enough that I generally give them as little personal data about me as possible.
Yahoo, on the other hand, still feels fairly safe to me. I'm willing to entrust them with moderate amounts of data (messenger, Flickr, customized My Yahoo page) because I don't think they're smart enough to combine it all in any meaningful way. Yes, that's security through incompetence, but in this case I think it actually works.
If MS buys Yahoo, I'll probably end up killing a lot of my Yahoo services but without any good alternatives. Suck suck suck suck.