I will share the short version of my experience with MacKeeper:
It used a redirect while I was opening my Yahoo email account to its Russian site. It absolutely and completely rendered my iMac unresponsive except for several huge popup windows telling me that I had (with a counter) 8,264 viruses. But they would be happy to rid my Mac of such awful stuff if I would only click here and give them my credit card information.
That was the only thing that seemed to be working on my Mac. I was unable to force quit my browser; I was unable to do absolutely anything. It was frozen except for the counting viruses and flashing click here button. The only way out: a hard shut down.
As a sidenote, this was shortly after the Resume feature was introduced and no one had as of yet come up with a way to turn it off (and Apple had not introduced a way to turn it off) So, every time I booted up the computer, the Resume feature obligingly launched the browser and immediately loaded the Russian site with its payment demand flashing at me.
Since there was no way out of the malware (yes, redirecting your browser and rendering it useless unless you pay them - in essence - a ransom, makes it malware in my opinion), I had to do a reinstall and, for safety, did a complete erase.
It took me more than 5 hours to get my Mac back.
No, I will not consider MacKeeper something that has a place on this planet (or any other).