If you're aware of racing games, you'll probably be aware of the Forza series. It's often considered to be Gran Turismo's direct competitor, although this is primarily thought by console fanboys.
Forza leans towards the simulation side of the sliding scale of racing games, but there's a lot of customization towards how sim-y. You can have ABS & TCS on, with a full racing line, damage off and automatic shifting on and have a fairly arcadey experience, or you can turn everything off and put manual shift on - which, which a good quality steering wheel, is almost unparalleled as to realism.
I'm somewhere in the middle of that sliding scale. I have ABS and TCS on, with automatic shifting, but with damage and fuel consumption on simulation and the difficulty up to full. I'm not the fastest by any means, but I'm usually in the top 20% times when putting the effort in. To improve much further would require getting to grips with all the intricate systems, and I just can't deal with that kind of commitment.
I also use the new Microsoft Wireless Speed Wheel from time to time. The game is significantly harder with it, but it's also incredibly satisfying. The wheel is far comfier than I ever thought it would be. It's very responsive too - you can turn the wheel a little and the car will turn just the right amount. And I do mean a little - the wheel is super sensitive. It feels great to drive with, even if it means giving up competitiveness. Although that might just be because I suck with it!
I'd like to mention Rivals mode. I'm not a particularly competitive person. I don't like to play against other people (indeed, even co-operative modes are a push). I feel like organizing matches or having to wait in a lobby to get into a race is a waste of time; it's too much of an obligation. Enter Rivals mode. What this is, is an expanded time trial mode. It gives you a track and a restriction, and the time of the person above you on the leaderboard. Beating that time gives you experience and money that is linked to your career profile. It's genius. More than once I've loaded the game intending to play the World Tour, but gotten sidetracked into Rivals by a message from the game that a club member has beaten my time on a track.
I haven't played it particularly long, but from the getgo I knew Forza 4 was something special. It's shed the tedium of Forza 3's career, linked everything together so everything you do is working towards something, streamlined car experience and thrown a shitload of content in there. It's fantastic, and I can envision myself playing it for a long time to come.