Their books are awesome, especially math books and sentence correction, but I found their CATs pretty useless.
Math is doubious, and by that I don't mean "hard to solve", but rather "hard to read". For me, hard math question is something you can read in 15-20 seconds and then strugle to answer in the next 2 minutes, not something that you cant even read under 3 minutes, being the case with almost 60% of
MGMAT CAT questions.
Verbal is even worse having too much some really weird assumptions... Horror...
When it comes to CATs, forget about anything that is not GMATprep (except maybe PowerPrep).
MGMAT is very strong in explaining advanced concepts and their math guides have outstanding coverage of topics that appear on GMAT.