I saw this when I was browsing through the Steam holiday game sale. It's currently on sale (until Jan 2.) for $3.49 so I figured I'd give it a shot.
http://store.steampowered.com/app/34860/
After playing a little while I must say it is a pretty neat game. The scope of the game is not huge, having 3 "open-world" maps (woodlands, arctic/winter, and desert/arid) and 10 vehicles. Each vehicle is modeled after a real-world truck, and although they aren't officially licensed it's pretty easy to tell what each one is supposed to be. If you are used to games like Gran Turismo with hundreds of cars and tracks the selections in MotorM4X may seem limited, but there is still enough variation to make it interesting. There are 4 race types; 2 are like traditional off-road racing, and the other 2 are crawling oriented. Trials presents standard gate-type crawling courses, and Pathfinder has a more open format, where you have to hit a number of checkpoints, but can choose any path you want to get there.
You can customize your truck with performance stuff like engine tuning and lift kits, and also visual stuff like paint jobs and aftermarket wheels. The performance stuff definitely affects the way the trucks feel and drive, and there is no real rating system saying one upgrade is better than the other (although the tendency is to believe the more expensive is better, just like RC!

)
The part of the game that really shines is the physics. The handling and articulation seem very realistic, and you have control of things like 2WD/4WD, High/Low gear, and F/R diff locking. These things all have noticeable effects, and are especially useful in the crawling events. The vehicles take realistic damage that is not just visual, but also affects how the truck performs.
As far as downsides I'd say there are a few bugs here and there, the graphics aren't spectacular, and the menu system is a little wonky and sometimes a hassle to navigate (no mouse navigation for the menus, really guys?). But overall for $3.49 it's definitely worth a try if you like off-road and crawling stuff. There are a few other decent off-road games out there, but I don't know that any of them have the realistic crawling features, and such accurate physics.
Final note: I would definitely recommend using an Xbox 360 controller or other gamepad for the best experience (or probably a true steering wheel controller would be best), but you could get by on the keyboard if necessary.
Here's a few pics and videos:
Race modes:
http://www.gametrailers.com/video/ra...motorm4x/34823
Physics and handling:
http://www.gametrailers.com/video/ca...motorm4x/34824
Terrain and weather:
http://www.gametrailers.com/video/ar...motorm4x/34825