I had an access to all MGMAT archives and used to work these problems during my GMAT preparation. There were some ridicoulosly difficult questions, you almost certainly will never encounter something like that on the real exam.
However, by trying to solve some of these monsters, I had to learn many useful concepts that have been tested on GMAT. This is the real strength of MGMAT's problems - they are not much useful for practice but they are extremely useful for learning advanced concepts.
On the other hand, from what I've heard, GMATclub Tests
(former Challanges) are even better tool, both for practice and for learning.
My GMAT experience
My retake experience - GMAT dropped from 720 to 690 - advice for retakers
My TOEFL experience - 108