I went and saw it in Cast previews today. While I'm not an American Idol fan by any means, and I didn't enter with high expectations, I still found it enjoyable. Not "great," but "good" at least. I can see how a lot would depend on the Guest talent. Thankfully, my show (2:00) had three very good people. Though one was better than the other two, they all did very well. It was surprisingly lightly attended, with about 150 or so people in the theater.
As others have said, the set was very impressive, as was the lighting design. There were great pains taken to make it feel like a real film shoot--even more than LMAX or Indy. The judges were pretty good, though you could tell that some of their comments were scripted. I didn't recognize anyone as being from the Adventurers Club, though I think one was from Comedy Warehouse. The host was okay, though just a little "wooden." There were lots of clips interspersed in the show from previous AI winners, offering advice and words of encouragement to the contestants. Some of them were mildly painful (IMO), particularly those from a younger AI guy--kinda clueless on the name, though from the crowd reaction, he's pretty well-known. There was also audience participation during a pre-recorded music video that was filmed in DHS. I think my favorite part was watching the judges interacting amongst themselves during performances or the interludes.
All-in-all, I'd say that this is a pretty decent attraction. I'm sure that American Idol fans will love it, and even those that aren't may enjoy it. I'm sure it'll improve as kinks are worked out and the Cast gets more comfortable in their roles. I'll probably pop in from time to time, but it's certainly not an every-time attraction for me. But, hey, it's a whole heck of a lot better than an empty building!