Best racing game for the Xbox, but that does not make it the GT killer.
The gameplay in Forza is over all pretty fun with a nice default difficulty not too easy, not too hard. However the higher you go in level the harder it becomes, mistakes really can not be made and the computer cars are very aggressive. The A.I. in this game is by far and above better than GT4. They do not stay on a fixed line, if you arent going fast enough they will go around you or knock you to mess you up. It can be frustrating at times as one mistake can screw you for the entire race. Your car will gain money at the end of each race and at certain points you will increase in level unlocking more events. I found this game to be a lot shorter than GT4, which can be up to you on whether that is a good or bad thing, I myself did not mind that. Not to mention the endurance races aren't incredibly long either which I liked. The tracks, I however was not really impressed with. There really isn't that many in the game, they just offer up different variations of them which to me does not count as a completely different track. There is a combination of actual and made up tracks in the game. I found the tracks that were made up, get boring very quickly, they are just not that fun to race on. I found myself saying "is this race over yet" quite a few times. I also thought that the races were a little long overall as well, not counting the endurance races. The online mode is the long awaited mode, with optimatch races, online career, and car clubs, and more. It adds to the replay value as long as there are users to race with and is pretty fun.
The graphics in Forza are about as good as they can get on the Xbox, they really aren't that much better than GT4 though. The car models I'd say are only slightly smoother than in GT4 and the backgrounds while not incredible do the job nicely. The crowd is heavy pixelated if you go right up to them showing repeated bunches of people, but that really isn't a big deal.
The sound in Forza is fine, with your standard engine noises and the like. The game comes with your standard generic rock, but does offer custom soundtracks which is very popular.
Forza Motorsport is in my opinion the best racing game on the Xbox, however the arcade people might find it a bit monotonous. I made references to GT4, it isnt any better or worse, but it just provides a different taste in the world of racing sims.