Hmm... well, GT4 is sorta a mixed bag for me.
First, I picked up GT4 and I had one of the EARLY first gen PS2s. Well, the PS2 promptly ate (and by 'ate', I mean it put track marks THROUGH to read side of the cd where the read head crashed) my first copy of GT4. One free copy, trade-in of my PS2 and a new PS2 the size of a credit card later, I am home ready to go for try #2.
Second, I had to unlearn ALL of the racing that I had done on XBOX Live and go back to the worst controllers on earth -- the PS2 controllers. Since I am waiting for Forza and its proprietary wheel to come out with 'force effects', I was hesitant to buy a 150 dollar wheel. However, after talking to 'The Hoppe', we discovered a USB to XBOX converter that would allow me to use the GT4 wheel (without 900-degrees of rotation) on the XBOX. This is a MAJOR plus as I hate paddle shift, but I love the sequential stick shift of the GT4 wheel + the GT4 wheel works with the PC (To be tested in the VERY near future once I get overnight parts from Japan)

. On NASIOC and 1-day later, I have a PS2 Logitec GT Force wheel for 90 bucks, plus the 20 dollar mail in rebate that Logitec is doing allows me to get the wheel for 70 bucks

If you like, buy.com has the wheel for uber cheap as well
Logitec Rebate:
http://www.logitech.com/lang/pdf/rebate ... _20-US.pdf
Third, using any wheel takes time. After finally getting the hang of it, I noticed there were three setting for the wheel in GT4: Amature, Professional, and Simulation. Due to the nature of me and AWD "hacked" driving, I went with the Amature or Simulation settings as they are the most 'fun' (of course removing all TCS and ASM settings from EVERY car). Something about Professional turns all the cars I drive into understeering pigs, even with +4 toe out in the rear, -3.0/0.0 camber splits and rear biased spring offsets.
Overall, I think that the AI on GT4 is rubbish. The Forza demo isnt much better as far as AI goes, but being STUCK with the AI and having NO online choice with GT4 is complete garbage. Any game, ESPECIALLY RACING GAMES, should all have some form of online support. The 'hacks' and GT4 servers I have played on are rubbish to as only 2 people can play with any amount of tolerable latency. Otherwise, LAN party it.. but you cant do that everyday.
So, I think GT4 is good for what it is -- an in-between game. I am going to be going Forza once it is released, and then everyone in STU will be safe as I will no longer be into autoXing and will be playing Forza 24/7/366.