I know I am different than 99.99% of coaster enthusiast... But I'd buy the Fast Lane deal online before I went to the park. The worst case is that you get there and you don't "need" the Fast Lane. But:
1.) It's money that (by the time you visit the park) has already been spent.
-and-
2.) Even if it only saves you 10 minuets... That's 10 minutes saved that could be spent on another coaster!!
Like I said, I know I am in the minority, but when we visit parks we ALWAYS buy the "line skip" in advance.