The problem isn't Heath Ledger getting replaced by a host of other actors - I really don't know why people are harping on that. It's like complaining about a dirty floor when the house is on fire. The problem isn't the other actors being swapped in - which in itself is actually somewhat interesting - the problem is the character they're playing is largely uninteresting. Also, there's something a bit odd about how they all seem to be playing their own standard selves, except that when you put them all together like that, the self seems to be a somewhat weak Johnny Depp imitation.
The real problem is that the movie is, quite simply, dull. An hour in I looked at my watch and thought glumly that it probably wouldn't be ending any time soon. After that, I think I was checking every 10 minutes or so, in vain hope that what felt like eternity had actually been a substantial enough amount of time that we were headed for the finish line. The special effects are neat, sure, but the plot is generally kind of inane. Which in itself wouldn't be so bad, if the characters weren't so utterly vapid. If you fail on both of those counts though, the result is generally pretty disastrous, no matter how impressive your landscapes.
The one exception to this is Tom Waits as the Devil - he's every bit as wonderful as you'd expect, and generally a pleasure to watch. He doesn't get much screen time, or even much interesting stuff to do, so it's not a case where the movie is worth watching for his performance alone - it's more like finding a cool patch as you're running out of the flaming house.