The Safe Mac:

The bottom line is that MacKeeper should not be used. The company behind it has shown a long history of unethical behavior, and the software isn’t worthwhile. If you have it installed, you should remove it immediately.

It always astounds me that these people are still around. I guess those relentless pop-under ads really work. Via Gus Mueller (via Dan Frakes) who notes that MacKeeper was breaking Acorn’s signature for a while.