I once likened them to semis on ice. Getting the car to rotate and transition between drifts feels like trying to redirect a glacier, and there doesn't seem to be much flexibility in terms of angle. Also, it's too difficult to find the right speed and keep from running off on the outside or inside.
I want to say NFSU2's drift physics were better, but that's not saying a whole lot. I'd definitely consider NFSPS's drift physics an improvement over the illogical travesty that were NFSC's drift "physics," though. I'm not sure how to relate GT4 to any of the others, considering GT4's flaws and strengths are so different from the NFS games.
In short, if you're looking for realism, look elsewhere. If you're looking for fun, look elsewhere, or play the other modes in NFSPS (the "grip" racing isn't too bad).