I have never really enjoyed Gran Turismo games but there is something about this one that means I can't stop playing!
The most obvious positive to mention (as with all GT games) is the absolutely breath-taking graphics which are unbelievable when first viewed. Reflections off car bodywork are perfect and the lighting of the environments is spot on. One of the best looking games out on PS3 at the moment - and this is not even the finished version!
The gameplay is also another plus, as the simulation level can be tweaked, so that even novice players like me can become Michael Schumacher in no time at all. I found it very enjoyable to play using my Logitech wheel as well so anyone who has one lying around I really would recommend wiping the dust off and giving it a try with this title (force feedback inclusion is the main reason for doing this - although at times it does go slightly hay-wire :-P )
Finally the easy to navigate menu system and the uplifting music (which includes some well known titles) are two more reasons to buy this game. However, as with most titles some negatives do occur - the main being the fact that in order to obtain a certain car (which is necessary for many races) repetition of some previous races to obtain "credits" is unavoidable which can get annoying - but not if you have the time. A minor niggle also arises with the AI but on the whole this is fine and should be much improved by the full release anyway.
Overall, I would say a good title to buy if, like me, you are impatiently awaiting the release of the illusive GTA IV/. It has a fair amount of life in it (1-2 months) and i may even return to playing it once I have obtained my copy of GTA - although i'm not sure this will be the case! :-) Definately worth the money as it is more than just a demo!