I took 3 Kaplan tests and 2 GMAT Prep tests. I didn't even know about this website until after I took the test, I wish I did! I generally do very well with standardized math tests, and I thought the quant sections on all of the practice tests were fair and there was only the occasional question that stumped me. On the real test I felt like so many questions were very difficult and I had to spend a lot more time on questions than I normally did. I feel like I can pretty well identify a challenging question from a moderate level question. Again, I thought that given the adaptive format, if there were so many difficult questions it meant that I was doing well, but that doesn't seem to be the case.
I don't have my official score report yet, only the printout from the exam. On the printout it says that the percentile rankings change every year so my ranking might change.