I took (if I remember correctly; I sat the exam almost a year ago) three practice tests, and I scored a good 50 points less on all of them than I did in the actual test. A lot of people, though, seem to have the absolute opposite experience; the stress of test day, which can never be properly replicated at home or in a library, really can have a big impact. I finished all the practice tests I took with some time to spare; in the actual test, I had about 20 questions unanswered when I ran out of time.
I found that the test I took was somewhat different from the practice tests in both format and content. The formatting stuff is (again) difficult to remember, but I think there was just generally a bit more stuff to read, and some styles of question I'd not seen before. It's very possible that because of nerves it just felt different, but I'm pretty sure that while the questions still asked you to do the exact same things as in the practice tests, there were some differences in formatting that slowed me down. I came out of the test feeling not that I'd done badly, but that I'd done less well than in the practice tests. I got lucky, however, in terms of content: the test I sat had lots of theory stuff on it, which is kind of my thing, so I ended up getting a decent score.
I guess I'd just say that while the test has obviously evolved since the practice tests were written, everybody who sits it is using pretty much the same practice material, so any changes are probably not hugely advantageous or disadvantageous to anyone. I'd mostly emphasise the importance of timing: I think this is probably the main thing you can control when it comes to any standardised test, especially such a purposefully gruelling one as this, so even if the practice tests are slightly different, their use value lies in allowing you to practice answering stuff quickly. I found the dearth of any written experiences except awful ones pretty worrying when I was preparing, so thought it might be helpful to share an experience that, while certainly not fist-pumpingly awesome, was kind of okay in the end.