If you like magic clockwork then you may very well want to see it just for the opening and the golden army. You'll probably also want to see it if you like the idea of a demon that was raised by humans to become a wanna-be anti-hero. His attempts to act tough are occasionally interrupted by the necessity of saving small children and kittens from danger, although you may or may not find this redeemed by his tendency to rescue such things and then continue fighting while cradling the rescued pet or baby in one or more of his appendages.
(Clearly if Hellboy is ever presented with a sadistic choice it will be along the lines of "Let die the woman you love? Or suffer the little kittens?")
In any case, the movie avoided a lot of the cliches and plot holes usually associated with movies of this type. shelleycat and i spent quite awhile discussing it and weren't able to find a lot of problems, and even came up with explanations for several of the things that seemed like potential problems. The remaining issues were certainly not big enough to spoil our enjoyment of the movie.
The big thing that they avoided and we were both quite grateful for was the whole trope of "The bad guy(s) have X-1 of the artifacts necessary to destroy/take over the world! But we've got artifact X, so we're safe as long as they don't get it. I know! Why don't we take that artifact to the place where they need to be assembled/invoked and challenge the bad guy(s) there?" =P
I do wonder why they didn't just turn around and go home after they got Hellboy healed, which seemed to be the ostensible reason for meeting up with the bad guy. However it's entirely plausible that they wanted to A: rescue the princess and B: kick the brother's ass, and it didn't seem like that big a risk since they didn't have the third piece of the crown with them, right?
Abe's decision to give the brother the last crown piece is a little questionable. Clearly he was emotionally deluded, but he knew that the prince couldn't hurt her without hurting himself, so all he could do was keep her captive. Was freeing an inviolable prisoner really worth risking the destruction of the world? On the other hand, we were kind of given the impression that he hadn't gotten any action in the entire period of his existence, and it's not like the brother who was intent on destroying the world was going to be well disposed to the plea that he let his hostage sister go out on a date :)
What does take a little more wondering is why did the princess retrieve the map scroll at all? Why not just leave it where it was and go into hiding? It's possible she thought her brother would figure out where it was, but then once she had it why didn't she try to destroy it immediately? At least she had the brains to try and destroy it once it was obvious she was going to be found, as futile as the attempt was.
I believe it was coraa who brought up the more interesting point, if Liz was able to melt down the entire crown at the end, why not just have her destroy the third piece when they first got it? I thought of the destroying the map/map case thing on my own, but i think i was too immersed in the "fires of mount doom" approach to artifacts to consider the crown pieces themselves.
A more serious issue which i did find myself asking throughout the movie was, if the princess was okay with them being executed at the beginning, why didn't she just kill herself then? A reasonable explanation is that it may be relatively easy to say "I am willing to die for this cause and will put myself in harm's way because of it" but much harder to say "I am going to literally kill myself right now for this cause." However granted that that might be the case and she wasn't able to work up the courage to stab herself when her brother first started chewing through the royal guard, at what point did she cross the line and decide it was easier to stab herself than to shout "Look out! He's got a knife!"? It also certainly would have been understandable if the prince had, for some strange reason, been about to stab Abe instead of Hellboy and she'd thrown herself between them and gotten stabbed herself instead, though that probably would have been a little more cliched. But it wouldn't have raised the question of "okay, why didn't you just do that earlier when it might have saved your father?"
Finally, in something that is more a question of character judgement than plot, coraa brought up the point in relation to sadistic choices that if a loved one is ever on death's door and the/an angel(?) of death(?)/whatever(!) tells you that if they are brought back to life then they will end up destroying the world, the proper response is "i don't believe you" or "i don't believe in destiny," not "screw the world, i want my lover back." coraa also professed the opinion that if she were ever in that position and neither answer A nor B seemed reasonable, then she'd rather not be brought back because hey, where's she gonna live anyways once the world was destroyed?
I agreed with her about that, but something i at least didn't really think of at that point is that it was Hellboy the question was being asked about. It's always implied that the destruction he will wreak will be some kind of demons-invading-from-hell apocalypse. Now when you or i open a portal to hell and summon a demon horde you can be sure that sooner or later (probably sooner) the demons are going to eat us too. However if a price of demons does it, well, the response of the incoming demons would probably fall somewhere between kowtowing and "hey thanks bro!" And after destroying the world they would probably throw a thousand year kegger, either on the ruins of earth or back home in hell. So certainly Hellboy would have a lot more options in such a scenario than any human you care to name. So it's not like she was saving him now only to doom him to die with everyone else, just dooming him to live with the responsibility of having gotten everyone else killed, if he even cared by that point. Not that he would necessarily appreciate that option if the choice had been offered to him directly, but it's not quite the same as if she'd made the same choice for a human.
All in all my only somewhat serious complaint was that the movie felt like it spent a lot of time building towards something epic, and when the final battle did happen it _was_ pretty epic, but it was also over rather quickly. It might have been a bit more awesome if the golden army could have come into play a little earlier and gotten a little more screen time, but i'm not sure how that could have been worked into the movie.