I don't think UA is the best in customer service, but I did receive such a letter from them earlier this year. When flying ORD-SFO on a 757, the aircraft developed engine trouble and landed in Iowa. The flight was delayed over 5 hours while a replacement aircraft flew in from ORD. The very next day, I received an e-mail from United, apologizing for the delay, and a couple of days later, a $ 100 voucher came in the mail. Of course, since this was due to mechanical problems, I had not sent in a complaint, and had no intention of doing so.