The non-spoilery bit: Well, this does assume that you've watched the preview, if you want to avoid even that much, then i'd just say this is a good summer action movie as long as you're not expecting another Spiderman.
When i checked RottenTomatoes.com before leaving for the movie, it had 22%. When i came back, it was at 18%.
Maybe it's just that i was set up to expect the worst, but this is one of the times when RottenTomatoes has really blown it. Normally i tend to agree with Ebert's reviews as well, yet he really blew it this time too. Looking back over the review, i have to think he was half asleep for a lot of the movie. He seems to find some things illogical that were actually clearly explained. He also gets really worked up about how unlikely or physically impossible some of the events in Venice are. I won't say what exactly the problems are because i'm being non-spoilery, but Ebert needs to get a grip, it's an action movie based on a comic book. Who cares if they fudged some of the details of Venice? I wouldn't have even noticed them if he hadn't pointed them out first.
All in all, the movie is FAR better than The Hulk was. Some of the artists complained about the quality of the special effects, but one of the others, whose opinion i happen to agree with, said that just about all the special effects were at the level you would expect for a comic book movie. Some of the action scenes get a little frantic, with lots of quick camera cuts that keep you tell from telling what's exactly going on (unless you have quicker eyes than i do) but overall they were good enough and the plot engaging enough that i was willing to forgive it.
It certainly didn't hurt that Mina (no idea what the actresses name is) looked hot in black, and i was always looking forward to seeing what cool powers she would unleash in the next action sequence :)
If you've read the comics and you're a stickler for detail you might not like the derivations they've made from the comic plot, however that seems to be slightly unfair considering the derivations the comics have already made from the original source materials :)
Oh, and there may be spoilers in the comments if anyone chooses to talk about it, so be carefull.
This section has spoilers! Don't read if you don't want!
When discussing the movie on the way to the theatre, i thought it seemed odd that they included Tom Sawyer. He didn't really seem like the type to fit in. The only other people without super powers are Quatermain and Nemo, and Nemo has an army and all kinds of gadgets. In retrospect, having forseen that to be the case, it should have been obvious that Quatermain and Sawyer would make a good master-aprentice type duo.
It makes sense now why they included Dorian Grey in the movie even though he wasn't in the books. Not only was the literary Dorian Grey nastier than most if not all of the others, he had a ready-made way to be blackmailed. I am a little bothered by the "just looking at it" being enough to kill him bit, although i can sort of understand the reasons. How did he figure out he can't look at it if looking at it would have undone it? And how did he know that he looked disfigured and withered in the painting if he couldn't look at it? Why did he have it hanging up on the wall in plain sight in that case? In any case, i guess they just didn't want to have to deal with the exact physics of what was described in the book. The scene when they found him is easy to describe/show in the aftermath, but whatexactly happened when he stabbed the painting? And could someone else kill him by damaging the painting? It seems like that must have been the case since he was able to be blackmailed with it.
How did Mina get to be a super-Vamp? She doesn't seem to have much difficulty with sunlight. Or with mirrors for that matter, although there's pleanty of leeway in the various legends and myths for them to maneuver around that one.
It was nice to see that the good guys were willing to kill people. Needless blood and gore isn't great, but neither is all the movies where the good guys manage to "knock out" all the bad guys, except for the main villan who they defeat, but then decide to let live, but then the villan tries to stab them in the back but accidently falls off a clif =P Not only are they willing to kill the bad guys in this, they're willing to shoot them in the back from 1000m away while they're trying to escape :)
I can't remember if there was anything else i was thinking of. Would be interested to hear the opinion of anyone else who's seen it though.