Despite the name, there are separate championship modes for both ATV's and MX, rather than them battling it out in the same race. Additionally, there's other types of vehicle thrown in here, such as golf carts, helicopters, dune buggies, monster trucks and planes. The ATV's and MX vehicles seem to handle the same even though you would expect way more agility with the MX and more grip with the ATV's.
The courses are full of bumpy elevation, and the bouncy physics mean you spend a lot of time in the air. The skill in racing is knowing when to launch yourself high in the air, and when to exit a jump early to keep your vehicle on the ground. You don't have to worry about using the brakes, just hold down the accelerator, turn when required, and decide whether to lean back or forward to change your altitude as you bounce around the track.
Whilst in the air, you can pull off tricks to score points. It seems purely optional though and is just a way of showing off. I saw no other benefit for doing so.
The Nationals Championship features a mix of themed courses from desert, snow, forest etc. The THQ SX series features indoor stadium courses, so each lap is shorter but you do more of them. After you complete a few races, you will be given a one-off race in one of the alternative vehicles (golf carts, helicopters etc.) to unlock them if you are victorious. The change in handling is hard to get used to, especially when you are given the helicopter without an explanation of the controls.
There's plenty of extra race types to try outside the championship including challenges, hill climbs, waypoint races, and quick race.
The game has one of those annoying menu systems where you select a file, select load, then it wants confirmation if you really want to load, then tells you that it has loaded correctly. If there's only one file, why can't it just automatically load it and only inform me if there is a problem?
The game supports an Xbox 360 controller but requires the use of the D-Pad in the menu, and the analogue stick when racing.
I felt the game should have had a tutorial to explain the more advanced concepts. Reading other people's comments you heard about 'pre-loading the suspension' and 'disengaging the clutch'. I tried these techniques from what I understood but didn't notice any performance boosts, so I was definitely missing something.
Other than that, I often felt there wasn't much skill to the game; you can play fairly casually and come out victorious. It didn't seem to have the intensity featured in other racing games. On the hard difficulty, the racers were more bunched up and the victor seemed to be decided based on who took the best racing line during the last few turns.
Since I never played the game when it came out, I have no nostalgic value attached to the game like most comments I see on Steam. To me, it seemed a fairly dull experience but I understand why people would have liked it when it was first released.