Thousands of people have used it. Apple's own staff at the Genius counters use it. Apple's phone support will also send users to the site to download and run MalwareBytes for Mac.
As Niel noted, it is not designed to be, or intended to be any type of preventative software. Its purpose is to find harmful software already installed by the user and offers to remove what it finds. It's still up to you at that point to decide to go ahead or quit.
The only real downside to it is that adware has escalated at a frightening rate. It changes daily. Some of it uses a list of thousands of random words to try and hide pieces of itself to avoid detection and subsequent removal. Because of this, MBAM won't always find everything. But the worst off you can be is it will miss something while removing other known junk.