I bought a copy ($40 at Best Buy) last night, and spent about 2 hours just playing around with it. It is stripped down "preview" version -- there are limited cars and tracks -- but man is it fun! I "bought" and raced an RX8 and a BMW 135i Coupe, and the first time I drove out of a tunnel and was briefly blinded by the glare of the sunlight, I knew this was more realistic than previous versions.
The cars handle very differently from each other -- OK, they were different from each other in previous versions, but the differences seem wider to me in GT5. BTW, if the real 135i coupe is anything like the GT5 version, it's going to be a fun car!
For the first time ever in a driving simulator, I experienced a snap spin after getting loose in a curve! I came in too hot, overcorrected and tapped the break (an obvious no-no in the real world, but one I get away with all the time in GT3 and GT4), and boom... an unrecoverable snap spin! Excellent! This means I can't just bash up against the wall with impunity anymore... I'm actually going to have to drive smoothly. Things like dropping a wheel or going too heavy on the rumble strips can really unsettle the car, so once again you are "rewarded" with driving smooth and keeping a good line.
The environmental graphics are beautiful, particularly on the Eiger Nordwand (Swiss road course near the Eiger mountain). The car's exterior modeling is clearly improved. This version doesn't include damage modeling, but that's OK with me. Sure, it'll be better with damage modeling, but I can live without that for now (I'd actually like to see them have damage modeling as an option). I haven't tried the "in-car" view, but I understand it's very cool to see proper gauges and your own arms in the car!
The opponent AI doesn't seem any better or worse than previous models, and the user interface will probably get a little tiresome (GT4's was pretty annoying too, with multiple menus to go through to actually get to a race). I'm guessing (hoping?) they'll fix those "challenges" in the full-version.
So I'll admit that this "review" comes after only about 2 hours of playing. Will GT5 Prologue’s cut-back content keep me entertained for the next 14.3 years we have to wait for the "real" version to come out? I don't know. But it's going to be fun winning races and buying all the cars the do offer, and perhaps give me a nice taste of things to come. Feels to me to be worth the $40.00!