Let me throw my 2 cents in. I work with 2 designers and use QB Customer Manager for both. The CM is very customizable - adding fields, creating different lists, etc. You can link to Quickbooks so at a glance know outstanding balances, track orders etc.
You can create letters/envelopes and this is tracked in the CM so you can print out a list of clients whom you sent letters to. It is a very functional program.
Just my 2 cents.