Forced to share a room, Reilly and Ferrell are at first at odds, then team up against the other family members, especially Ferrell's obnoxious and ridiculously successful younger brother, Adam Scott. Scott's verbally abused wife is Kathryn Hahn, who for no good reason is sexually obsessed with Reilly. The antics of Reilly and Ferrell infuriate Jenkins until he leaves Steenburgen. Ferrell and Reilly are then obligated to become respectable to coax Jenkins and Steenburgen back together.
How others will see it. Will Ferrell comedies are proven money-makers, and Step Brothers did turn profitable despite the presumably outlandish salaries for the film's leads. The movie drew mixed reviews and was mostly ignored by the festival circuit. At imdb.com, the movie has a very high 230K user votes. User ratings decline with advancing age of the viewer, from 7.2 under 18 to 6.2 over 45. Nonetheless, these are reasonable ratings for what amounts to a stupid comedy.
Most viewers either love the film or hate it. Those who love it admit that it is stupid, but don't care. Those who hate it know it is stupid, and can't get over it.
How I felt about it. The most obvious criticism is that Ferrell and Reilly are too old for their characters. I know that they are supposed to be 40, but Reilly (for example) was 43 in 2008, and is not credible as a man-child of typical intelligence. Both are miscast, and it would have been better to replace them with slacker twenty-somethings and change the script accordingly. This would also allow them to cast parents who aren't supposed to be senior citizens.
The next criticism to make is that the film is more uncomfortable than funny. Stupid actions tend to have serious consequences, and the movie makes light (yes, I know it is a comedy) of events that could kill or permanently injure our misguided heroes.
Another problem is character inconsistency. When the stepbrothers get their act together, near the end of the movie, it doesn't seem possible, given how stupidly they have behaved for the previous hour. Richard Jenkins has had enough, or hasn't he; he's unhappy with Steenburgen, or isn't he; Reilly and Ferrell hate each other, or do they; Kathryn Hahn wants to leave her husband for a much older and uglier loser, or does she; Adam Scott wants to torment Ferrell and Reilly, or does he; etc.
So, we have an implausible and mostly unfunny comedy based on a premise that the director is too cowardly to continue to its proper conclusion. If Reilly and Ferrell are unwitting anarchists determined to alienate their parents if not completely wreck their lives, they shouldn't instead become money-making conformists running a partnership. And, of course, Kathryn Hahn should not be throwing herself at homely dufus Reilly, no matter how big a jerk her own husband might be.