i've taken 4, 5, 7, & 8 so far (6 and 9 this week)
4 had, by far, the toughest scale. got my lowest score there. also, remember that 4 was administered "back in the day," under the old format, so it's probably (hopefully) less of a reliable indicator of what you might score on the real deal.
i took 5 early on, and thought it was manageable.
i thought 8 was good - VR on 8 a bit tougher than on 5 and 4, but no different than the EK passages.
7 messed with me a little bit. i scored 2 pts lower on 7 than on 8 and actually found PS and VR on 7 to be a little bit more challenging than on 8 (anyone else feel this way too?) - more random, stupid calculations and 'percent by weight' crap on PS that i never really studied and weird questions in VR (the passage with the 'high road/low road' corporation stuff had some question stems and answer choices that i thought were out of left field).
i also took the test straight from the AAMC booklet, where the VR passages and questions were front/back, rather than on facing pages, which i found annoying. i didn't want to keep flipping back and forth when referring back to the passage and so i was relying more on memory to answer the questions. lame excuse, i know - but did anyone else feel that way about the VR on the printed AAMC 7?
anyway, if i had to rank the tests i've taken so far, in order of difficulty:
5 < 8 < 4 (because of the scale) < 7