It is absolutely worth it. There are actually a surprising number of scenes that were shot using IMAX cameras, and it is without a doubt the most immersive film I have ever seen.
Heath Ledger is pretty much a lock for Best Supporting Actor at this year's Academy Awards. Granted, there are still about 6 months until Oscar season, but if you look at what's come and gone, and what's in the pipe, there isn't exactly a lot of competition. Not to mention that his portrayal of the Joker is absolutely amazing... I honestly couldn't have imagined a better version of the character if I tried.
Go see it. And I hope you enjoy it as much as I did.