Well as someone who is right into sim racers and has been prowling the forums i am sensing big things from GT5, and i don't think i am alone. I have noticed some BIG advertisement ramping up on GTPlanet.net from the likes of Ferrari, Bmw and Porsche to name just a few.
Also, the price of the top steering wheels (G27 & Fanatec) has been rapidly climbing and they are becoming harder to even get in some locations.
I think these are all signs of something big, and i'm pretty sure the industry knows that.
I expect GT5 to clear GT4, and perhaps even challenge GT3.