The plot is that thousands of years ago, Mankind made war against the creatures of magic. In the end they were defeated when the Elves and Goblins forged an invincible mechanical fighting force called the Golden Army. But the war was ended when the creatures of magic recoiled at the horror of what they had wrought and created a truce. Now an Elven prince seeks to break the truce, gain control of the Golden Army, and use it to destroy mankind.
Along the way Hellboy runs across fairytale creatures that are quite Grimm. Think of the dark fantasy world in Del Toro's prior work Pan's Labyrinth. The fae and folklore behind H2 have a similar edgy and sinister feel. The BPRD gang also gets some character development (unfortunately except for Jeffrey Tambor). The action scenes are solid. Although the story isn't based directly on a comic like the first movie, it still has the Hellboy comic feel that makes the movie fun.
If you liked the first Hellboy movie, you'll enjoy the second. It's a good ride and definitely worth a few bucks.
UPDATE: Chris Byrne has this to say:
What bugs me the most though, is that the quality was there, what they really needed was more time to explore; and they could have had it. The movie was only 110 minutes with credits; and audiences in this genre are fully prepared to sit for 150 plus credits. If they had taken the 150 minutes, this movie would have been spectacular.I disagree. I like unplumbed depths in my films, especially when good pacing means not plumbing them. Hellboy II is a fun action film, but would make a horrible epic. We need more 110 minute movies made today, because making them generally leaves about 40 minutes of crap on a cutting room floor somewhere.