Cheating and sometimes overly agressive AI really drags down the overall fun.
Once they failed to do all that my car mysteriously would run wide into a turn allowing the car(s) behind me to pass, no matter how many times I had taken these turns perfectly, if a car was within 1-2 lengths I would run wide without fail. (This I like to call the Professor Xavier AI mentality X-Men fans will get that!) Also if you lose the lead forget about it, much slower cars gain speed seemingly from nowhere and are gone giving you too little time left to even get close.
Overall:
Many cars to choose from and many ways to upgrade them though kept simple and straight forward with explanations on tuning them makes it fun to do so. Beautifully rendered tracks as well as cars brings a nice feel to the game overall. The damage could have been more realistic, slamming into a wall @ 160+ and the front bumper just swings around isn't anywhere near "simulation". Other than the AI's steroid abuse this game would be superb!
Arcade:
The increasingly difficult AI brings the fun to a quick and violent end. The very last track takes approx 10 minutes per lap with a Class S car, driving any other car is just a waste of 30 minutes or more. Not only does the AI increase with new tracks it does so within a race as well. If you can manage a large gap between yourself and the second place car (provided you can manage to make it to the lead and retain it) the evil AI will make your car float out of the corners taking away much needed time. Many times you'll start a race and notice a car you've yet to unlock blowing past you boosting your blood pressure.
Career:
Increasingly difficult but as long as your car is tuned to your liking you should do well. There are many different racing formats but all stem from the same rules, win or lose and win to unlock new toys! Purchasing new cars and winning new cars/parts is always fun but managing to do so in the latter part of the game is hard. I mainly use my career cars in other modes and had TONS of fun customizing my cars' paint / decals. This is where I think most of the fun in career mode is.
Free Run:
The most fun mode of the whole game I would say, though you need a friend sometimes for competition! Lack of cheating AI is both a relief and a disappointment here. If there were something to race against you would feel much better about winning, then again spending hours tuning your car just to have a Mazda Miata blow past your Supra would just make you hunt down the programmers.