The FFB quality in GTS is very dependent on which wheel you use. My cheap T150 is actually well supported by GTS and the force-feedback is OK, and it definitely got better since the release version. It's definitely not as detailed as PCars2, but if you enjoy online racing, the overall gaming experience in GTS is much better.
However, my new Fanatec CSL Elite wheel has absolutely terrible support from Polyphony. There's hardly any feedback at all with this wheel in GTS. Even though I hate all the bugs in PCars2, I'm actually going to play that more now, until Polyphony improves the Fanatec wheel support in GTS.