I have posted my complaints about the demo in another thread, however, having spent some time in the demo now (with the correct settings) I will say that it is a significant improvement over GT4 in the physics department.
However, it does seem to me that oversteer is still underdone, braking is too easy & weight transfer is minimally represented, so that it still feels very much in the GT tradition. It's the exact opposite of Forza 2 which continued in the vein of Forza 1 with very tricking braking, little understeer & massive oversteer. (I actually wonder if part of the sense of the braking being too easy in GT, comes from no tire marks on the pavement, which makes it seem like you're floating thru the turn, whereas in Forza you leave rubber on the road all over the place.)
As far as the cars in the demo are concerned, I think my favorite is the BMW. The GTR is a big disappointment - fast, but so smooth & well-mannered that it's not very exciting to drive. It would have been really nice to have had a powerful RR car in the demo so that we could really put the "improved physics" to the test...
My comment about the G25 in the demo, is that the steering feels very light & not much FF, as others have said. I really like the steering options in DiRT (the demo, I don't have the game) where you can set the "weight" of the wheel as well as the FF - it would be nice to have this option in GT5/P.