I noticed that the 2008 edition had a lot of errors. You can just look at the errata list and fix those, however. (I wish I had known that before!) Most of the errors were such that they completely baffled me when I would read them, which I do think affected my studying since they slowed me down.
Maybe they fixed those errors in 2009 edition. So it might be worth getting for that reason.
However, if you've already annotated your 2007/2008 edition, then I say forget it! That was what happened to me, so I just stuck by the 2008 edition. But if you haven't really marked up the book with juicy notes, then I'd say definitely buy the 2009 edition. Honestly, the book is a steal. I'd buy that book even if it were $100 or $200.