It's the shiny lure of interaction, and this campaign isn't about getting new fans in to see Batman's latest film. It's about warming us up to the new Joker.
Jack Nicholson is a tough act to follow in any role, and even for those who didn't care for him as the Joker - Heath Ledger (pretty boy with talent) was going to have an up hill fight to win a place in the fans' hearts as Gotham's most notorious mass-murderer.
Instead of showing up the theater muttering about a production still of the Joker's makeup test, fans will be giddy over seeing the mastermind himself on a story-tall screen! This campaign builds a relationship between the fan and the villain - it establishes a tie that will help the die-hard, mild-mannered, and mediocre-intersted all see this Joker as THE Joker.
Sure it's whimsical at times. Some of the clues might seem to lead nowhere for now, but that's part of the fun. The Joker isn't supposed to make sense.
And in the games themselves, we're not working against the Joker (an interesting twist really) we're working FOR him. He's leading us around by the nose, and there's a sense of entanglement that comes with each successful solution. Whatever he's planning, as a participant, you are a part of it.
When the final curtain rises, I think we'll all be pleasantly shocked and horrified at the madman's game, and while you can't say this was a financially feasible campaign until the box office numbers are in - I'd say it's an exceptionally successful one if the fans (new, diehard, fringe, and other) look up at the big screen and don't see Heath Ledger in facepaint and a purple suit. They see the Joker laughing back at them - THAT is success.