After putting in hours upon hours with Forza, both with the pad and with the Fanatec wheel, my vote is still for GT4. Despite being a GT fan (not fanboy...) since day 1, I was actually looking forward to another commendable console sim or at least a comparable driving experience. Forza's just not it. If it wasn't for the eclectic car selection, and relatively impressive AI, I'd put it behind TOCA 2. It's got the online play, and the damage, and the customization, yadda, yadda, yadda, but it feels "off".
The cars feel stiff and wooden, not intuitive, or connected to the pavement, for that matter. It's even more apparent in the replays; the cars hover above the tires and the suspension, totally unaffected. They don't pitch or yaw or lean under braking or cornering, at all.
Then there's the sound. I run a Dolby 5.1 system, and while it's not a 10,000 watt earth-moving setup it holds it's own and Forza sounds like crap. The car engines and tire squeals are there, but it sounds like their coming out of the same channel; or all five channels at the same time, like a racing soup or something. It's difficult to tell how close your opponent is or what side he's coming from.