I will mince no words in saying I am a fan of the Gran Tourismo franchise. Played GT2, GT3 and 4 heavily and now comes the precursor to the full version, GT5: Prologue.
Let's look at things that are fun, easy to navigate menus, fairly quick load times (mostly owing to the game data being installed on the HDD versus run purely off BRD). Environments feel much more immersive and subtle touches like lighting/shadow and vehicle detail are are times nearly photorealistic. Car response in terms of steering and control feel a little bit easier to manage, particularly when trying to draft the AI cars. Some of this however is due to the longer throws of the analog sticks on the Sixaxis/DualShock 3.
The downside. Let's face it, as games go Prologue is a preview. Granted one that I'm paying for, but hey I'm a nut. I think it provides a good initial feel for the type of handling and graphics you'll find in the full release in 2009 (Painful I know) but many areas need to be optimized. Lag during online gameplay (though truth be told I had similar lag issues on Forza 2), occasional stutter with reflection rendering, lack of tuning and damage physics give you that slightly less than true GT feel and gives an edge to the likes of Forza.
Overall if you just want some slightly different tracks or, like myself am a GT fan, I think the $40 price tag isn't great, but not horrible. Visually stunning, fairly good handling and enjoyable track variation and environs bode well for this teaser release and hopefully results in a better final product in GT5.
Final Score 7.9/10