If you want a proper racing simulator buy Gran Turismo 2 for PS1 and enjoy and don't touch this crap even with a mile long stick.
Gran Turismo will have more vehicles, REAL endurance races (instead of drive around 4 minutes), a way better customization and tuning abilities (more later) probably more tracks, better soundtrack, better voice acting (i.e. silence which is better than a phone-sex-line-granny-trying-to-be-sexy "Welcome back John" every time you open the game), better driving mechanics (more later), pit stops etc. etc.
Once again Codemasters make it look like a nice game but after a while you'd give away your aunt if you could just remove it from existence.
Mechanics are something inbetween Grid 1 and 2, meaning you won't be powersliding (i.e. slowing down) all the time but anyway you'll be doing that quite a bit. I can somehow deal with it when driving a street or touring cars but FFS I DID NOT START A FORMULA SEASON JUST TO DRIFT AROUND.
The open wheel cars have just the same amount of grip with a little bit lower centre of weight and more power so they'll slide just as much as your Ford Mustang or Nissan Silvia but they may be recovered from it a bit more easily. Fun for a while or for arcade racers but not a part of a "simulator".
If using a keyboard to drive you need to have traction control on and thus (I suspect it's due to it) all other drivers are faster than you in the start.
If you try to ram the opponents you end up eating mud yourself every time because they're like moving brick walls on rails. They don't give a SH** about you and your pushes if you are not in with a total kamikaze, also you have a great chance to end up spinning out every time you hit someone.
The penalty system for cutting corners is complete utter bullcrap. In a same circuit you could drive 100 meters on grass and it doesn't do anything but in the next corner when you hit the kerb a bit too hard and touch the grass you'll be slowed down for something like 7 FÜGing seconds. The AI doesn't get penaltied and happily push you out of the track if you're too close so enjoy!
Depending on how far have you played in the career you'll have zero to a little possibilities to fine tune your vehicle. The changes are the exact same as in Dirt 3 but in Grid you may only have two options for each, 0 or 100%, depending on your team. This pretty much renders the tuning useless because unless you're looking for more sliding you aren't doing a thing with a possibility to set 100% of your braking power to rear or front tires.
I also noticed that either something is terribly wrong with the open wheel cars or the game has differential lock tuning inverted, locked diff should make you slide even more but in the game open diff causes it.
If you can still remember the fine tuning in GT2, I can tell you it was real high-tech. Don't even dream about something like that.
All same group (or class) vehicles are basically the same to make them even. When you have two team offers the XP is the only thing you need to check, your team mate will always suck and the car is just a moving image with exactly the same abilities as the other one.
There are something like 5 different cars in each class (except the formulas which have 1 or something) so basically you have a few to choose from but it's nothing like the hundred cars in Gran Turismos.
Along the basic races there are some other modes such as time attack (or something) where you have X laps to score the fastest laptime (so it's basically a qualifying but called race) endurance and drifting.
Drifting was surprisingly pleasant and pretty cool but the same mechanics (meaning same controls but -100% grip) were seen in Need For Speed something like 10 years ago. It's also ridiculously easy, within a lap you can score 1,5 million points and AI scores 500-800k.
Endurance races are about driving the circuit for X minutes and your tires will wear out. In addition to the "normal sliding" which is rather heavy you cannot brake or turn softly with a keyboard so you'll eat up your tires pretty quickly but it's fun enough. No pitstops though, would have been cool.
Multiplayer is... well, online multiplayer. The lobby is plagued with an automatic voice chat which you have to switch off every time if you don't want to use it.
Codemasters have a ghost car mechanic which makes everyone ghost cars when in a corner to avoid collisions and pushing but it's not working very well as it makes you visible when you're not braking heavily and thus you're again open for hits which means there will be hits just like there wasn't any ghost thing at all.
I have really mixed feelings about this. I enjoy arcade racing but this isn't it. I also enjoy simulators but this isn't it either. It's not completely bad and I've had fun moments but it's not anywhere near the addictiveness and fun I had with Gran Turismo 2 when I was a kid. It isn't a best comparison but it's the last good racing simulator I've played.
If you enjoy semi realistic racing and can swallow the crap they'll send you with it, you'll like this.
If you're in for a good simulator or action full arcade racing, you won't find either in this one.