Q: does mackeeper really work

No. It is not worth installing MacKeeper. It is a scam product.

OS X Mavericks/OS X Yosemite:

If your Mac runs slowly:

http://support.apple.com/kb/PH13895

Startup in Safe Mode

http://support.apple.com/kb/PH14204

Repair Disk    Section: "Try Disk Utility"

http://support.apple.com/en-us/HT203176

Reset SMC.     http://support.apple.com/kb/HT3964

Choose the method for:

"Resetting SMC on portables with a battery you should not remove on your own".

Increase disk space.

http://support.apple.com/kb/PH13806

Run Etrecheck and look for  and  remove third party apps that  may slow down the computer.

https://discussions.apple.com/docs/DOC-6173

No. It's pure garbage. Avoid it at all costs.

If you want to wreck your Mac while simultaneously wasting time and money, yes MacKeeper will work.

Determine what's wrong and fix it. Do not use MacKeeper or any other so-called "cleaners", "doctors" or "medics" unless you want to compound your existing problems with more.

Please describe the problem in as much relevant detail as possible. The better your description, the better the chance of a solution.

For example, if the computer is slow, which specific actions are slow? Is it slow all the time, or only sometimes? What changes did you make, if any, just before it became slow? Have you seen any alerts or error messages? Have you done anything to try to fix it? Most importantly, do you have a current backup of all data? If the answer to the last question is "no," back up now. Ask if you need guidance. Do nothing else until you have a backup.