The UW self assessment came closest to predicting my score as well (7 points under the real thing). I took it 3 days before the actual exam. The NBME that I took a week before the exam underestimated my performance by 20 points.
I liked being able to review the answers and I think it was money well spent.
the UW assessment is very accurate. their score was 4 points higher than what i got on the real thing. the NBME exams are also very accurate as well, but the NBME probably are more +/- 10 points.