I played this game back in 2011, then I found the game on this site, and instantly bought it.
Why? Here is a few reasons..

About the game:

Before you start racing, you can choose your sponsor and vehicle, and after confirming your car, you can change a lot of things about it, here's a few things that you can change: gear ratio, downforce, differential, brake bias, ride height and suspension. If you crash you can use a "flashback" to rewind. If you complete the race, you will earn XP, and every level up unlocks more sponsors.

The game is a simulator game, with multiplayer and singleplayer. You can play the game on the PS3, Windows, Mac and Xbox360.

The main marketing for this game was that Ken Block, the famous rally racer was in this game.

Game modes:

There's a lot of game modes in this game. Some of them are fun, some of them not so much.

rally: here you race against the time, not other racers (you race against other racers time), a racer starts every 10-15 seconds. Your goal is to complete the racetrack faster than the others.
Trailblazer: My favorite game mode, it's the same as the rally, but with wider roads, and way faster cars (some of them can go with 200km/h or 124 mph)
head 2 head: 2 racers race against each other on a track, usually they start "face-to-face". The winner who crosses the finish line first.
gymkhana: You need to to tricks (drifting, jumping, gate breaking, etc), your goal is to get the most points in a set amount of time.
rallycross: The racers start at the same time, your goal is to finish first.
land rush: The same as rallycross, but with "harder" terrain.

Multiplayer:

Its really fun, especially with friends. There is a lot of game modes just for multiplayer (you can play the "normal" modes, in multiplayer too). There is Capture the Flag, Invasion, Outbreak Zombie, Cat & Mouse.

My favorite was the Zombie mode, where there is 1 infected, and the other players
had to run from the zombie(infected), if they get hit by the infected, they became a zombie too.
In the CtF (capture the flag), there is 2 teams, and 1 flag, every team has their own base and the goal is to get the flag to your base more times, than the other team.
In one lobby 8 players can play.

Graphics:

The game looks really got for its age, the only problem that I have with it, is that there is multiple same looking people standing next to each other sometimes, but it's not that big of a deal in my opinion.

Driving:

The driving of this game feels good, isn't that hard to play, but it is harder to master (drifting around a corner in rally etc.)

Sound/Soundtrack:

The cars sound really good, also there is a lot of great soundtracks in the game.

My thoughts:

My friend had this game on steam, so I bought it (not only because of him, but for nostalgic reasons too). I 100%-ed the first season (there's 4 seasons, each seasons have 4 events, every event has 5 to 12 races), which means that I finished first place in every race that season. We didn't played that much multiplayer together, because his internet wasn't good enough to play, and talk on discord at the same time. When I played DiRT Showdown, I really loved the drifting and I was really good at it, but in this game it wasn't that easy, I was happy to finish 3rd on a drift event.