If you want to see (roughly) how they stack up to Kaplan/AAMC in terms of difficulty, here are my scores in chronological order.
AAMC #9
Summer
36
13
11
12
Kaplan FL7
12/12/07
31
9
11
11
Kaplan FL6
12/17/07
37
12
13
12
Kaplan FL4
12/18/07
38
13
12
13
GS #1
12/21/07
32
11
9
12
GS #3
12/22/07
39
14
12
13
GS#2
12/24/07
35
12
10
13
GS#4
12/26/07
35
9
14
12
GS#5
12/28/07
35
12
12
11
I think the Physical Sciences curve is brutal and some questions are poorly worded (the 12 in GS#5 for PS corresponds to four missed questions). Verbal reasoning is too easy but the curve is harder than AAMC(37 = 12). Biological sciences tends to have a lot of detail-oriented questions related to anatomy (be prepared to answer questions pertaining to loss of vacuum in the interpleural place, etc). Also, they tend to reuse PS questions from test to test somewhat which can get annoying (particularly, the fact that half-filled orbitals are unusually stable comes to mind). Once I take another AAMC I'll have a better idea of their scoring accuracy