We were last there in 2015. We went twice. Once in 2014 and once in 2015. Spring time for both visits.
At least when we were there it wasn't overly crowded. The lines are typical theme park lines and they didn't move quickly, but I think the longest line we waited in was for their coaster, and it was maybe 30 minutes.
My kids spent a long time just looking at the lego construction area. And they LOVED watching the water ski show.
I don't think the front of the line passes would be worth it unless you're going at a crazy busy time of year.