We went on Mother's day weekend, and called that day to make a reservation. Online it said they weren't taking reservations, but we were able to do it over the phone. It was fairly crowded when we got there, but we probably only waited about 10-15 minutes to get seated. The moved people along fairly quickly.
If you can make a reservation online now though, and it's somewhere you really want to go, I'd say do it now. We love it because our DS is a little dino-phile, and I get gift cards as rewards through mypoints.com!