When I was in customer service in the software industry I liked almost all of my customers. There were only three or four bad ones in about twelve years.

The people I really hated were the salesmen who told them that the software would do stuff that it wouldn't. They would even sell a product using programmed demonstrations to show the customer all sorts of wonderful things that the software just couldn't really do (unless they had a team of programmers with years of experience.)

I also hated the developers who were designing CAD software without having spent a day on the drawing board. They hadn't a clue about the needs of a skilled draftsman.