I'll make my review short not to spend too many words on writing about what you have already known. I only say two things; `Supercross' is about the world of Supercross competition, and very short (80 mins). If I may add one more thing, you don't find anything else in the film.
`Supercross' stars two young actors Steve Howey and Mike Vogel as two brothers who race motorcycle, but as you know, the film is made primarily for the purpose of showing the Supercross action scenes. So if the film is virtually full of clichéd items such as brotherly jealousy, father-like mentor, or big competition that concludes the film, it is perfectly OK with me. I actually do not care if one character gets badly injured during a race and is confined to bed, and the same person shows up again, only several days after the terrible accident, walking and even hopping on crutches as if it's only an ankle sprain.
What really matters is the race scenes, which need more improvement. Don't get me wrong here because I am not talking about the races or the stunts themselves. Stunt-turned-director Steve Boyum shows a lot of shots of the Supercross bikes flying in the air, but he shows alarmingly little concern as to how to present them to us viewers. The camera overuses the same angles and the same techniques (like slow motion and fast-forwarding) over and over again. The shots of the dirt race course and the audiences (both real ones, I think) themselves are shot fine enough to convey the feeling of watching the real race, but as the director shows little skills in narrative, we have to listen carefully to the announcer's voice to know who is who, or who is winning.
Those things deprive the film's actions of the dramatic tension of the race scenes. Watching a flying bike may be a really exciting experience, but you need more than that to make the race truly thrilling and exciting. After all you can watch the real ones at the Supercross competition, so why make a movie?