I'd say that the best way to find the right amount of realism, that's still fun to play, is to start of very realistic and then see how diffcult it is for somebody new to get the basics.

Then add some assistance or reduce the effects that would happen if it were completly realistic, so that the player gets more time to react and see how his actions are effecting the gameplay.

I usualy have the best experience when I'm surprised how realistic the controls are, but still able to learn it in a reasenable amount of time.
How realistic it should be in the end should depend on how importend the vehicle is for the game, in my opinion...

so long...