I have few enough customers that everything is in a encrypted excel spreadsheet and I use Trello for tracking.
However, one of my clients though has a ton of clients himself, and he is extremely picky about software being easy and the features he likes. I know he has tried a bunch in the past, but loves http://www.nutshell.com/ Anytime a call comes in, he has that up so he can see everything he needs from past conversations and add in all new.
I liked the flow of it, just doesn't suit my needs.