As a certified Mac support professional for 5 years (not as long as many, but long enough) that has seen too many innocent/unknowing users tricked into installing and even paying for MacKeeper, I would strongly discourage anyone from installing MacKeeper or most anything that purports to accomplish what they claim. It's a pain, but can be uninstalled (after diligently searching online for good step by step instructions because I don't trust the maker's uninstaller).
Perhaps there are some that feel they benefit from having it installed, but it's not well written (can cause crashes, freezes, perhaps corruption of data), uses unethical advertising (poses as something recommended by Apple Inc., persistent/annoying pushiness, charges a lot of money for benefits that can be accomplished with much less or no money).
I work in the Apple Mac support industry. So I see a lot of problems. I don't believe Mac's don't have issues, because... well... people typically only come to me when they're having a problem with their Mac, whether just perceived or an actual issue. But I can say that I've seen this software too many times and it's only been trouble.
If you want real help to speed up, protect, clean, etc. your Mac, try these first:
1) Make a list (mental, on paper, or some typed out smartphone/computer file) of any anti-virus, anti-malware, anti-spyware, cleaning, etc. software you may have installed.
2) If you're close enough to an official (not just a 3rd party Apple Authorized Service Provider) Apple Store, make an appointment at their Genius Bar (maybe make 2 or 3 appointments back to back if you think you'll need more than 15 minutes), back up your computer (if you know how), and take the computer in and ask your questions and present them your list. If you haven't backed up your data or don't know how, then ask them to help you with that too. These appointments are free. They don't know everything, but they're free and they can (often) be helpful. If they don't help you to your satisfaction, then you can ask them to recommend nearby 3rd party Apple Authorized Service Providers that could spend more time helping you (which may not be free). At least the Apple Store Genius Bar appointments are free and they have no problem referring you elsewhere if it may help you more. Genius Bar support and 3rd party service provider "scopes of support" overlap, but they each have their purposes and advantages.
3) Find a 3rd party Apple Authorized Service from the previous Genius Bar appointment recommendations or from http://locate.apple.com/ (and click on the upper right "Service" section, choose "Mac", and enter your location).
4) Call them and ask (or just take your list there) and express your concern that your Mac seems to be running slowly and if they can help clean it up. If you haven't backed up your data or don't know how, then ask them to help you with that too.
Many recommendations as to how to best maintain or clean up your Mac will be personal preference, but I can confidently say to everyone: don't bother with MacKeeper. Even if you're not one of the many that pays MacKeeper money to cause more headache, don't take the risk. There are safer and better solutions.
~ Ben