From what I have heard from friends and classmates and people who have taken the MCAT is that they are harder than the actual exams. They are definitely more difficult than the AAMC ones.
I think they are good practice though, and worth it. Verbal is quite difficult, and some PS/BS questions are challenging, but overall, it's doable. Don't be discouraged if your scores on these aren't where you want them though.
I would love to hear from other people as well, but that's my 2 cents.