First thing is first, yes, the Steam version of the game does not come stock with Ferrari or Porsche cars. This is a licensing issue and Steam is not trying to screw you over. However, you can easily return Ferrari and Porsches to your game by updating it and installing the No-CD patch (Note: Steam version is already No CD, you still need the patch though to get Ferrari and Porsche to work).
On to the game. It's a very old game, true, but the physics are outstanding and with mod added content like cars and tracks you'll find the graphical experience to be pleasant enough to not be distracting and when you're in the moment it's very easy to forget how dated the game looks. The AI is like all AI: terrible. They can run real life lap times and generally speaking they're fairly competent -but- they are absolutely terrible at racing off the racing line. This isn't unique to GTR2 or the GMotor engine, it's an industry wide issue. It's AI, you deal with it, and it's not too bad at the end of the day.
Other things to note:
1. The FFB is amazing once tuned properly for your wheel. It's horrible if it's tuned wrong. Take some time to set it up right.
2. If you're using a controller, you'll find that the game is just as enjoyable and just as playable
3. If you're using a keyboard, you can STILL be competitive with the right sensitivity adjustments. The game is that versatile, but obviously your experience will be the best with a wheel.
4. The 24H day/night cycle works as intended and is gorgeous. Sunsets blind, nights are dark.
5. The weather system leaves something to be desired. There is no dynamic weather to speak of, though there is a mod that corrects this. When the tracks are wet it's more like ice and more often than not TCS isn't enough.
6. Most importantly, this game has a STRONG modding community. Is there a major track out there you want in your game? Someone has created it. Is there a series you want to race in? Someone has created it. The only major series absent to my knowledge are the latest models of NASCAR, but this is a road racing game so what do you expect?
I figured I should also mention how it compares to other games. The next generation of sims is coming out as we speak so it might seem weird to pick up an 8 year old game. I feel that Assetto Corsa is likely going to be GTR2's spiritual replacement (Not that I have anything against pCars, or that there is anything wrong with pCars) in terms of the "sim" quality and possibly modding community. But it isn't there yet.
As it stands this is IMO, the best sim out there that you can buy overall, and it's largely thanks to the modding community. If you don't feel comfortable modding, then it's still a great sim with the 2004 GT championship, but I would advise picking up AC or pCars instead (once they release in full that is). The maturity of the mod community is what makes GTR2 what it is today, because they've pushed the Gmotor engine to its limits. Being said, AC looks better, handles better, and has arguably better FFB. It's too early to say how the career mod, modability, and AI are in comparison.
If you're comfortable modding, then this is and will continue to be the best sim out there until either AC, pCars, or rfactor2 builds up its mod community. If so, buy the game and head over to nogripracing.com and see what I'm talking about.