The only way to ascertain this imo, is to look at things like amazon, or hmv. where the people who buy the game give their own opinion. thats how i look at reviews. imo you can always tell how good a game is by looking at the number of amazon reviews. for example Black Opps got around 8s and 9s, but is about 3 stars (so 6/10) on amazon.com.
those types of reviews end up being from hundreds of people. and they speak volumes to me. and i think GT5 will end up being at about 4 - 4.5 stars, which is the same as any triple A game. its a racing game, as long as it does that i dont see what the problem is.
having never played GT before, (but planning to get this one eventually) i think its just personal perference, its being reviewed by people who are really critical of the series because they've played it before. i mean its not like other racing games have any type of real time damage, so its just because its GT imo - its got very high expectations from the gaming press.
