The explanations can be shotty at best - sometimes there isn't really an explanation. However, like others have said, the questions are much more difficult than what are on the AAMC FLs. The questions are very detail-oriented (since, as SN2ed mentioned, they are designed to review what you just went over), which isn't typically what the MCAT is like.
That said, it's good to practice on the hard material. It makes the FLs seem so much easier.