Stepbrothers was pretty good in my opinion. Definitely not the funniest movie I've ever seen, and definitely not something everyone will enjoy, but great nonetheless.
The acting was great, it was a nice pickup for Will Ferrel especially. I was pretty impressed by the many moments where no lines were being said, but the characters' feelings were clear through only their facial expressions.
The parents also did very well, I was pretty surprised when they became a little more extroverted and actually yelled and interacted with the brothers as adults, as opposed to parents.
My only gripes were, the sleepwalking thing was a little unnecessary. It really did nothing for the movie but push the dad overboard, which could have been much funnier if done another way.
Also, I didn't think the movie did very well separating the moments that needed to be serious from the moments that were funny. You have this huge emotional scene where Brennan sings an opera song, and then it's broken up by Dale doing a drum solo.
Finally, I didn't like that some of the problems went unresolved. The dad never buys a boat, and Derek's wife strangely does nothing the last 15 minutes of the movie.

I gave it 8/10