The pairing of Ferrell and Reilly should have been comedy gold, unfortunately Step Brothers only manages comedy bronze. Having said that, Step Brothers has a lot going for it and if you allow yourself to give in to the pleasantly retarded proceedings, you'll love it.
The step brothers in question clearly have a ball improvising and just being silly as they happily bring their own take on Dumb and Dumber to the 21st Century. In contrast to the chaotic buffoonery involved, Jon Brion provides a beautifully orchestrated, subtle score which manages to fit in very well even though it is somewhat out of Step Brothers' league.
Overall, Step Brothers may not be the single most memorable comedy you'll ever see but it's a bunch of fun for anyone willing to shut their brain off for a couple of hours, sit back and lol.