I saw the Brothers Grimm, and thought it was ok but not great.
I agree that it is dark. It has lots of twisted imagery, and some pretty disturbing things happen. (But I will not give any spoilers.) It isn't the typical fairy tale, and although there are references to some fairy tales like Red Riding Hood, Rapunzel, Hansel and Gretel, etc. they are mostly very quick and not very much connected.
A constant theme in this movie is that there is resentment between the brothers, because the younger one, Jacob (Heath Ledger) had too much fantastic imagination. At the beginning, instead of buying medicine for his sick sister, Jacob buys a handful of 'magic beans' and since then, Wilhelm (Matt Damon) thinks he is a stupid dreamer, and always is criticising and putting down Jacob. The theme of the brothers overcoming this difference could have made a good story with a moral, but I thought it was overpowered by the special effects and too much weird and dark images.
I thought the cast was ok, but not great. Matt Damon and Heath Ledger were not bad, but I didn't find them very good either. A week later I don't remember their performances. I did not like Lena Headey as Angelika, the heroine/love interest. For most of the movie she played the character as unpleasantly gruff and hard. Maybe she was trying to make the character wild and independent, but I thought her performance was unlikeable. Monica Belucci was very good as the vain but beautiful Queen Mirror, and most other important roles were played by character actors who gave good performances but the characters weren't likeable in most cases.
I like movies where I care about the characters, but could not do that with the Brothers Grimm. The humor is dark and though I admit I laughed a few times, it is definitely not a light-hearted humor and not for children. If you liked movies like Tim Burton's Sleepy Hollow, though, it is probably worth seeing this one in the cinema, for the special effects.