The most convincing simulation ever, off-road, on-road, driving, or otherwise.
The environments in which you spill mud are all rendered as photo realistically as any racer I've seen. The sound is spot on. To be honest, none of that matters much. The gameplay, physics, and feel of the drive is what separates it from any game other out there. It's not just he most realistic rally game, it's the most convincing simulation ever, off-road, on-road, driving, or otherwise.
That's pretty much all I can say. The physics are amazing, but there's no way I can really explain how the game drives with using text.
Download the demo, even if you're not interested in racing. It's free. it's readily available.