I've been a Technical and Corporate Recruiter for 20+ years now off and on. Monster is the big 800 pound gorilla. I did get a contract job off it.
That said it is not perfect. For example the basic search for a recruiter by location is by State. Thats fine for smaller states, not CA. It's been 6 years since the first time they agreed with me and said they were fixing it. Searches start at the top of the pile and work down by the search perameters being used. So if you are a programmer in CA you rapidly move down the stack with all the other Programmers in CA. You might be out of the usable search range in a week or less. If you are a widget cleaner in Oklhoma and worked for the only company using widget cleaners you might be in the range for a year, except if the one company isn't looking Oh Well.
For the searcher it is good for linking to company pages. If the job you are looking for is relatively rare and the available labor pool is large then your chances are not good numbers wise. I would refresh my resume periodically to get it back up to the top of the stack.
Realistically - I wouldn't look for some kinds of folks there. Would I look for new grads in non-technical fields, non-skilled labor, lower-level blue collar trades. New grads I can find easier at Colleges locally, the other categories tend not to be on the internet, although the numbers are getting better. If you are a hs senior looking for a burger flipping job, don't bother. (I've seen some, they look like they are illiterate and expecting the moon.)
Do some research and see if there are job boards for your career field. They seem to be better than Monster for most. Monster can be too big a lot of the time.