We went a couple of years ago and bought the Express Pass. Not really worth the money in my opinion. As macraven said, depends on when you go. We went to IOA in July, during the middle of the week, and the lines weren't that bad. In fact, there was one time my sister and I got off DD, decided to go on again, and were on it again right away. Didn't even wait 2 minutes. Same with Bluto's Barge. Only time we found the EP beneficial was when we went on Hulk. We went on Hulk first thing when we got into the park. Later on in the afternoon, we wanted to go again and it was an hour wait; that's when we used our EP and only waited like 20 minutes. We went to US the following day and it was pouring on our way there. As we were walking in, there was a lot of people walking out. Rain stopped about an hour later and it seemed like we almost had the park to ourselves. Our wait time in lines averaged about 10 minutes.
If you're planning on staying at any of the US/IOA resorts, your room key works as an EP. Only difference is, your room key gives you unlimited EP privileges, as to where the actual EP itself only lets you once on each ride. We're planning on going again in a couple of years, and this time I'll skip buying the EP. Of course I won't be going in July either. I'll be headed there in the off season.