I've never had any direct experience in dealing with the "customer service" departments of any headphone manufacturers, but have had a lot of great experiences with headphone-related gear, most of which was through head-fi.
Top class, all the way, but in no particular order:
JMT who built some portable amps and mini cables for me. What a great introduction to head-fi!
Mikhail at Single Power Audio and Ray Samuals at Emmeline Audio, both of whom are now freinds of mine. But it all started with me as the customer and each of them caring very much about what I was looking for. I lump them together only because they're both such great amp builders, true gentlemen, and easy to get along with. I think of them as twin sons of different mothers!
Todd the Vinyl Junkie and Larry at Headphile.com. These guys are also great to deal with. Larry (Mr. Woody) is really a cool guy. He can make any cable or headphone-mod you can think of and has a ton of patience with customers. Todd is also one of those guys who is always available for whatever your need.
Jan Meier of Meier Audio. I've had several great transactions with Jan, and he recently agreed to serve as my "agent" when I bought an Orpheus system from a guy in Germany that I didn't know. Jan arranged everything for me!
Koji at EIFL Corp in Japan is also highly reliable and will give you the best price every time.
Ultimate Ears was so-so. Nothing too exciting either way. Some people have reported being treated like rock stars, but my experience was that they were polite and helpful, although not always efficient (i.e., did not let me know when the product had shipped or provide any tracking info as I had requested).
I've not had any "bad" experiences, although Ultrasone was pretty useless when it came to answering my questions. It took them months to respond to emails.