Their website[1] looks awesome, I don't own a Mac (Windows user) but if I were to buy one I would definitely consider buying and using it. It's good that this site[2] is the second result on Google for "mac keeper" though.
Edit: I saw this quote on the Mackeeper website you linked (it is good design) "MacKeeper was noticed at Macworld by a journalist from Cult of Mac". So I chased that phrase and all I found was algorithmic page generators like this http://drgogek.com/is/is-mackeeper-really-a-scam-cult-of-mac....
Using a different tack CoM mention Mackeeper in a single page found from their search box, an affiliate promotion page ... using Google unearths more promotions, https://www.google.co.uk/search?q=site%3Acultofmac.com mackeeper.
So, it looks like their quoted reviews are from people they're paying to sell the product, that's a warning sign.
Didn't investigate further.