During the point in the day where Banshee's line was 4+ hours, we took two rides on it within 15-20 minutes. Aside from that we rode EVERY other coaster that offered FastLane and didn't wait more than 1 or 2 trains with the exception of Flight of Fear because it broke down. But once it came back up, I was the first train out.
If you're not a local, and you only have one day at the park, it's WELL worth it! Really, it depends on the circumstance. I'm not sure I'd recommend Fast Lane to someone who lives 10 minutes away from the park and knows when to go to find the queues empty, but for me, who flew in from out of state, I couldn't imagine NOT getting Fast Lane.
i agree 100%, days like this are the exact reason for Fast lane