I love Gran Turismo, loved it since day one, and have never missed a title since. Its a fine line between realism and arcade, and thats why it sells so well. I hate NFS:Underground and anything after it, the previous titles are cool though, driving the M5 and being chased, good times!
Ive only got the demo, and i reckon you should just keep playing that for now, as you're probably aware LFS isnt complete yet, but the level of gameplay in the demo, is more then enough to convince me! At the end of the day, it doesnt matter what Tom Dick or even Harry has to say about the game, its your money, and your time, if you like it buy it!