I think we both thought it was an interesting but not enthralling movie. Perhaps part of the problem is that we were watching it at home (which is common) while poking around at other stuff (which is less common) and couldn't quite keep track of exactly what was going on (which is even less common.) Finally about two thirds of the way through i pulled up the wikipedia page on it and got caught up to where we were in the synopsis, which helped with figuring out the ending.
I'd actually kept track of the fact that he'd made a deal with the devil about collecting people/converts/souls/something. However i also picked up that Parnassus had "won" the first bet and wasn't sure if there was another bet going on, and if so what it was about. I'd also somehow gotten the impression that everyone who entered the tent counted as a "win" for Parnassus and that they all got sacrificed/killed/whatever, which made what he was doing seem rather more sinister than it actually was.
I did like that the trailer gave a different impression about who the good guys and bad guys were than was actually the case in the movie. At least to the best of my recollection anyways. Of course the movie itself also did a reasonable job of clouding the issue on that account. Although that might just be that i was unduly influenced by what i'd been lead to expect by the trailer, or that i wasn't paying enough attention and missed out on a couple motivational clues, or both.
So all in all i don't think i'd call it a great movie. Certainly an interesting albeit somewhat confusing one, which really just means it fits in well with a lot of the rest of Terry Gilliam's work. (cf. Time Bandits in particular =)