I heard that, and it lacks a structured career which makes it pretty boring, and the fact that a few of the cars are completely full-on perfect while others are a lot less detailed and a lot of the cars don't even have interior views. And the exhaust notes are supposedly pretty crappy, too. I had GT5 Prologue back when I had a PS3 and it completely blew me away graphically/physics-wise, it's a shame that simulation racers are going downhill so fast. Forza and GT are the only ones left really, and neither of them are even half the games they could be.
I've always liked GT more in the sense that it appears more to car enthusiasts, while Forza appeals more to the rest of the gamers who might have something of an interest in cars, but nothing more. GT has such a more tasteful and memorable selection of cars, while Forza simply crams in all of the most well-known supercars and imports.
GT3 and 4 were my favorite games ever, period.