OXM has given 10/10's to the following games on the 360:
OXM's only 10's
Fight Night Round 3
Gears of War
BioShock
Halo 3
Call of Duty 4
Mass Effect
Grand Theft Auto IV
So I'd atleast beg to say that GTAIV is in very good, 90% territory. The lowest-scored game of that list was FNR3.
Back from the dead, I'm afraid.