I can think of no better description for what I've experienced in Gran Turismo 5 Prologue. The graphics are amazing - the 1080p Gran Turismo experience I've been waiting for... and Gran Turismo 5 Prologue is just a sign of better things to come; it is merely a very, very playable demo. Gran Turismo 5 should be bigger and better. Prologue is just to wet our appetites.
The user interface has been massaged quite a bit and, although it's not release-ready just yet, it looks like Gran Turismo 5 Prologue will redefine gamers' expectations for level of polish and production quality.