Ive had 1,2,3,4 and now 5 of the gt series. They are unbeatable as a simulation and ive spent so many hours on GT5 over the last 2 or so years, however there seems to be a lack of fun at times when comparing to some street racing games such as Midnight club and the earlier need for speed's. Obviously these games do not have the longevity or proper phyics like gran turismo, but they have some good features like the customisation, free roam racing etc.
It looks like Forza is probably the most complete game on the market. Such a shame its on 360 as i hate xbox controllers and i will never get one.