If you don't have plans to see this movie, you can check the spoilers here and then come back.
This was less a full-formed movie and more a series of bits that Will Ferrell, John C. Reilly and Adam McKay didn't have room for on Funny or Die. The physical comedy is top notch and some of the scenes and lines are hilarious, but there's no real coherent plot or flow to get caught up in, placing Step Brothers for me a notch below the real comedy standouts of the last few years like Wedding Crashers, The Hangover, Superbad or even Knocked Up. There's nothing wrong with watching an hour and a half of funny folks do mostly improvisational skits, but that's what I've got Saturday Night Live for. Adam Scott breaks away from the pack as far as giving his douchebag brother role some actual meat while Richard Jenkins and Mary Steenburgen shoulder the bulk of the dramatic load as the titular brothers' put-upon parents. I have nothing really critical to say about the movie or about what Ferrell and Reilly bring to it other than it's somewhat lacking heart and while I'll probably quote some of the funnier lines, I have no real desire to re-watch it. Honestly, I get the sense that the cast had more fun making this than I had watching it.