There's no doubting that this is, by far, the finest looking racing game around, but there are still a few technical shortcomings. The London track in particular suffers from some pretty noticeable screen tearing. Some of the effects used in replays look a bit unpolished as well. These will hopefully improve for the main GT5 game. Or, maybe Polyphony will go crazy with the downloadable content and actually update the graphics engine gradually.
IGN has only sampled Prologue through a few races and events so far, but it already seems like a production worthy of the Gran Turismo name -- at least as much as the Japan-only Gran Turismo 4 Prologue was prior to the full GT4's release.