I still think buying Opera, and adopting/adapting would be a good thing for Microsoft. It would give them a major playing hand in the mobile browser market too.
From Opera's perspective, they might gain a lot of users overnight, which may or may not be a good thing. But i do think Opera as a browser is the neatest, most feature-rich and yet unbloated piece of software i've seen. And they deserve a bigger desktop market share than they currently have.