My experience has been pretty positive. The timeline:
Preordered a Turbo S Standard Edition. About a day later, I changed my mind and upgraded to a ClubSport Edition preorder. I used the contact form in their web shop and submitted my request and received a response in about 48 hours with instructions on how to pay the difference using PayPal.
When the Turbo S ClubSport Edition arrived (meeting ETA estimates. I know Fanatec has missed ship dates for a lot of people, but mine arrived roughly when expected), my 6-speed shifter had a rattle like there was broken plastic inside. I submitted a support request via the web form again and got a response within 48 hours that they'd ship out a replacement. They did not request the defective one back. Received the replacement in about a week.
I received a RennSport Wheel Stand as a gift, and mistakenly ordered the shifter mount for the RennSport Cockpit by accident. Ordered the correct part, then submitted a request to cancel the wrong order, again using the contact form in the store. Didn't hear back within a day, and was concerned about the order going through, so I bugged them again and got a response later in the day that the incorrect order had been canceled.
I've since ordered other random accessories, ClubSport Pedal Tuning Kit, a Carrera Final Edition. They arrived in working order and on time, so I haven't had to contact customer service since. In all cases when I did, I provided detailed info and always referenced my order numbers. I haven't used their email support and never called, so I can't comment on experiences there.
Hopefully Fanatec has hired more CS staff and implemented a proper CRM so that everyone can have consistently good service. I get the feeling if you have technical issues during the 2 year warranty period they're happy to help you, but often drop the ball and fall behind regarding customer service. I don't think it's malicious. Obviously if you're on the receiving end the motivation doesn't matter; you just want your products fixed in a timely manner.