If you're there on a crazy hot day, you'll probably do alright without FastLane. We had some VERY hot, sunny days a week or so ago and the park was not overly busy. Basically if it's a day where most won't want to be outside (TOO hot, rainy, etc), the park won't be too busy. I'm told that day the lines for Leviathan were as short as 10-15 minutes by the evening.
If you have the extra cash though, I would highly recommend FastLane. Even if the park is moderately busy, it will make a HUGE difference as most coasters have poor capacity. Other than Behemoth and Leviathan, lines don't move very quickly.
If you enjoy water parks, I would recommend purchasing FastLane, get a couple laps in on rides during the first hour the park is open. Get to Splash Works as soon as it opens. Lap the water park then head back into the park and take full advantage of cutting the lines.