Webmin is actually a decent, free, open-source control panel. I have used it several times before, and I would use it again if I needed a control panel for my own purposes, or for another sysadmin. It certainly makes it easier to change configuration files and add mail accounts, etc. You don't have to login to the shell, and you can access it from any browser, anywhere. The best part? It's free. Install it and forget it.
However, I don't know if I would use it for customers, as they typically like flashy, more intuitive control panels, which is understandable considering their purposes.
I hope I could be of reasonable assistance to your efforts.