I once saw a coworker accidentially installing it and it surely is a nasty piece of malware. The problem is that one can never be quite sure whether every piece of the software was removed, which is why we ended up just reinstalling OS X and avoid guessing.

Since then, a few of my friends, who aren't into tech, almost installed MacKeeper. They all had a long history of using Windows and felt that installing some Anti-Virus, Anti-Malware junk was the most natural thing. For normal users it's really hard to distinguish between legitimate applications/websites and shady stuff.