I play the f1 mods on rfactor every day, used to play f1 challenge, both games are simular and developed by the same people, these games are very realistc in handeling but the ai is not as good as gp4. You have to set your car up to make it slow in some corners to match the speed of the ai, monza for example is very bad because the ai have high downforce on the straits, not sure if i am doing something wrong. Its still way better then gp4 imo because gp4 really does feel like its on rails like someone said. in rfactor with a good wheel you can "feel" your way around the bumps and learn the handeling.it simulates the physics well where as gp4 simulates a race well and the atmosphere.
Actually some of the best mods are not the f1 mods because the ai speeds are much better with out the downforce levels to play with one example is the Historic GT & Touring cars mod.
Here is a list of the really realistic mods for rfactor that are worth downloading.
F1 1988 carrera
f1 1991 mmg
f1 1991 le
Grand Prix 1979 (f1)
F1 2005 CTDP
F1 2006 CTDP
F1 2007 MMG
FSONE 2008 (f1)
The last 5 f1 mods are really good
cART Factor 1.00
CART Factor: Prologue 1.01
2007 Champ Car World Series 1.20
2006 Champ Car World Series 1.50
Gp2 Series 2006 1.00
LMS Limited Edition 1.06
Historic GT & Touring cars mod
Touring Car Legends 1.10
VW Beetle Race 2.11
VW FunCup 1.15
ETCC 1.20
Historic Rally Cars (HRC) 1.10
Rallycross 2.11
Rallycross Group B 1.00
Trucks Series Racing 2.00