I would say the sciences on the real test are a little bit easier than the practice test. The non-sciences on the other hand, seem to be about the same or a little more difficult on the real thing.
Practice:
Bio - 390+
Gen Chem - 340ish
Org Chem - 350ish
Physics - 320ish
QR - 390+
RC - 350-370
Real:
Bio - 400
Gen Chem - 390
Org Chem - 370
Physics - 340
QR - 320
RC - 300
I found that the QR questions on the real test, while not entirely more difficult, took more time than the problems on the practice tests. Because of this, I felt uncomfortable and had to rush myself, leading to a score that was much lower than what I had anticipated.
For RC, on the real test a lot of the questions were set up like this:
Statement A + Statement B
a) Only A is True
b) Only B is True
c) Both are True
d) Neither are True
While this might not be that much more difficult, it was not something I was expecting, again causing me to feel uncomfortable and simply pissed off at Kaplan for not having questions like these on the practice tests. These questions also take more time as you have to look back to find the correct answer for two statements rather than one.
This is just my experience and while it might not reflect other's, it still should give you a better idea of what to expect.