I asked Mr. Google and he told me this:
"MOUS (Microsoft Office User Specialist) Certification - now being called MOS Certification"
http://www.examnotes.net/MOUS-certification.html
I always knew Office was so bloated that you needed to take a course to manage to use it correctly
Joke apart...
I don't know whether the exam itself is hard, but I don't think it is because a lot of people seem to have Microsoft certifications nowadays. Whether you could get a bigger income with it is another story. IMO if I was an employer I'd much more favor experience over certification. Or skill whatsoever : if the guy
shows me he's able to do something, I trust him more than the other guy whose only the diplomas tell he should be able to do it. That sounds logical... but unfortunately it's not often the case. I'm one of these folks with ZERO certification in computer engineering (my diplomas say: electronics) and I had quite a hard time to find a potable job related to computers.