I think Ella T. said it quite well in her review @ http://www.villagevoice.com/2009-12-22/film/heath-ledger-s-ghost-haunts-the-imaginarium-of-dr-parnassus/ "This is potentially wonderful, if not exactly new stuff, but Gilliam and McKeown's willful refusal of coherent narrative and determination to pack every idea about art they ever had into one scenario, make this fiendishly gorgeous movie more exhausting than exhilarating to watch."
I am a huge Gilliam fan, and I love his loyalty to free-minded art, but he consistently runs into a problem with some of his films in which he has so many artistic visions that he concentrates on the troubles of smashing them all into the film, while the pacing and good solid storytelling get pushed to the side. This movie is chaotic and whips along from scene to scene so whimsically that the characters stay caricatures, and the story is imbalanced, and the audience is not emotionally invested in any of it. The actors were capable, but they were just ripples in the water of this movie, not having the chance of leaving much of an impression. If more care had been given to developing a real journey into this film - something with growth and depth and thought, instead of just being a flit through dreamland with the Kook family, it would have been amazingly wonderful. I wanted to like this movie, but its shortcomings are just to vast for me to do so. Overall, a good try.