We saw the HP exhibit in Times Square last week (7/30/11). It was $26 and annoyed me right away as the entrance line goes through the "pose here with your family" spot. HATE having to have a photo taken first!
This is not a roam-around museum. They let you into the exhibit in large groups, and you basically follow everyone from exhibit to exhibit, staying more or less in line. The beginning is very crowded, frustrating, and pushy, but it gets better as you go into the exhibit- the crowds thin out and you actually have a chance to see the items on display. There are LOTS of costumes from the movies, as well as other items like wands, lists, eggs, beds, snitch, time turner, etc., all identified by name and episode. There are displays from herbology, the potions dungeon, etc.
Normally I wouldn't have done this kind of thing in NYC, but admission was included on the NYPasses that two of our group had purchased (one teen and I). I guess it was a good exhibit. I started out annoyed, and the crowds at the beginning didn't make that any better. The three teens I was with were enchanted by the whole thing. I wish I'd skipped it and given my pass to my daughter to use, to save the $26 admission fee. There are discount cards available around town (found them at the NBC studio tour ticket counter) but they are not valid on weekends. I am a pretty avid Potter fan, but I wouldn't do this tour again (although I think my teen might).