Screw the critics, Jonah Hex didn't suck. That's not saying it was a great movie. There are some similarities to the Wild, Wild West (time period, President Grant, a super weapon, and a villain with a similar goal, but a different motivation). But Hex was better.
I didn't read the comics so I don't know how much they changed stuff or if the movie was true to the comics but its a good guess that a lot was changed. However, Josh Brolin, when I could understand him, made a pretty good Hex. Megan Fox just looked good, her line delivery was lazy and her Southern accent sounded more like Southern California. I also think John Malkovich had a hard time with his Southern accent too. The trailer looked cheesy and a lot of the descriptions in the reviews sounded horrible, but when I saw them on film it wasn't that bad.
Maybe if they had made it more of a gritty Western it would've been better, or made it R-rated, or removed or toned down the supernatural elements, or took out the super weapon, but I don't think those things really made the film horrible. It was a decent film. Not a travesty like WWW.