2 things:
1 - Blame Sony for not sending out review copies sooner. They've been in this business long enough to know that people will try to have reviews up the second the embargo is lifted. If they felt that you needed a week to review this game properly they should have sent out copies sooner.
2 - It depends on the genre of the game in my opinion.
A racing sim is different than something like an RPG or action adventure game where you really should play the game until the END to give a proper judgement. I don't have GT5 but judging by past driving games is the experience you've had after 2 days really that different than the experience you'll be having after a week? Sure they might not have enough time to fully play with some of the extra features like the track editor but I think things like inconsistent graphics quality, confusing and cumbersome menus, quality of driving simulation, opponent AI etc are all thing you can judge based on 2 days of play time.
