Yeah, it's bad. Pretty terrible, really. Some spoilers included, but I'll keep it vague.
The story is a hodgepodge a various Hellboy comics, and the movie is at its best in the first half when it plays like a series of vignettes of Hellboy having these crazy adventures. I mean, it's still cheap looking and goofy from the start, but for the first half it maintains an enjoyable B-movie feel, like the action movie equivilant of a Nightmare on Elm Street sequel. It's once we pass through these few short stories worth of material to enter the main storyline (we're halfway through the movie before we introduce the rest of the main characters) that the film goes from enjoyably bad to just bad. It feels as though the third act of this movie was cut down from something originally twice as long, as we violently ricochet from plot point to action beat with no connective tissue between them and you feel like there was a tremendous amount left on the cutting-room floor, making it feel like a mediocre collage of footage with no oomph or focus behind anything. What's funny about once the movie gets into the plot heavy part of the movie is that despite pulling directly from Mignola's comic it hits pretty much all the same story beats covered in Guillermo del Toro's previous two movies. You'll be amazed at how similar almost all the sequences in the third act are to scenes much more satisfyingly realized in those previous films.
The cast is pretty game, and try their best with the bad material and dialogue given to them. It's easy to imagine a much better movie with pretty much the same cast. McShane as Broom is maybe the most interesting, as he's a great and charismatic actor yet the character has been rewritten to be so thoroughly unpleasant and unlikable so it all cancels out. His main affectation is that he drops f-bombs in every sentence and is pointlessly mean. Those obscenities were maybe intended to be funny (?), but they don't amount to any memorable zingers...
... That's the weird thing about this R-rated business; it seems like an afterthought. All the swearing and gore feel like they were plopped in at the last minute and none have any punch to them. There's no swear words providing awesome moments like Samuel Jackson dialog in Pulp Fiction, and there's no gore that legitimately surprises and terrifies like the eye-gouging in Fulci's Zombie (to bring up two effective examples). The dirty bits are totally perfunctory and this could easily be a PG-13 movie without losing anything of value.
Oh, it's very cheap. Some of the worst CGI you will see this decade, and a lot of the practical effects are bad, too. At several points you see Hellboy's horns wobble! There are a few well realized practical effects and neat creature designs in there, though.
The Baba Yaga, an incredible character from the comic based on Russian folklore, is reduced to a latexy Goosebumps villain. Lobster Johnson, another great character here played by great actor, is reduced to a pointless cameo who will no doubt completely flummox general audiences.
In short; it's a total systems failure of a movie. It's vastly inferior to Guillermo del Toro's movies, and it falls so woefully short of the comic books on which it's directly based that fans of neither will have much to like.