There have been quite a few forum discussions about Flex, Eli Krantzberg has posted some interesting tips and links here in the forum, FAQ 8 links to some more information about using flex, and here is Apple's own resume:
When 9 first got out many people thought that it was a small upgrade because you can't tell the difference from 8 unless you look very hard. As you can see from the other responses this was not the case. Logic 9 is a big upgrade and one of the most significant maybe ever. Flex have been mentioned (and it works) but also version 2 of the take folders. When you get the hang of using them there is no going back, it's just so fast.
There's also quite a few new plugins where the guitar simulators and pedals are the most impressive I find.
Of course if you never use plugins, only do one takes and never miss a beat you may just as well stay with 8, and also should Logic 10 suddenly come out of the blue in 2 months it doesn't make much sense either.
Also one other thing, if your computer is old you will find that Logic 9 makes your machine work a little harder. Not much but enough to drain an ancient G5 that could just about get by in Logic 8.