For what reasons is Microsoft not a great place to work? I may be biased because my brother works there full-time and a friend is interning there but both of them very much enjoy it.
I don't know anything about Yahoo!, but my perceptions of them are still positive. If you have an awful work environment, people will just go elsewhere. especially if you're qualified to work at either company.
This would be more how their perceived by hobbiests and people who work at startups. For people who want to work at large companies or are willing to work at established companies it doesn't seem true.
Personally, I've only heard negatives about working at MS from people who have been there for a while, new employees are happy and excited to be there.