I loved Step Brothers as much as I hated Taladega Nights. Convinced that John C. Reilly and Will Ferrell were a toxic comic combination, I only watched Step Brothers because I was in need of something light, cinematically speaking.
I'm glad I did, as it turned out to be a wholly enjoyable experience, and the best comedy I have seen either actor participate in. Granted, it could definitely be considered a "stupid comedy" along the lines of Billy Madison or Tommy Boy or Dumb & Dumber, but inasmuch as you like those movies you are likely to enjoy Step Brothers.
See it if nothing else for the fight sequence that follows Will Ferrell's character putting an artificial pair of testicles on John C. Reilly's character's drum set. And if the oddness of that statement alone does not sell you, you are obviously not for sale in the first place and you should just return to what you're doing and listen to Smash Mouth's "Allstar," the song I despise most in the universe.