I'm a HostGator Reseller. A related note is posted on their forum.
I tried WHMAutoPilot a couple of different times back around the beginning of the year. It seemed overly complicated and I eventually gave up and kept billing manu-matically. I looked at MB and some of the others now and again but nothing really stood out.
Then a few days ago I tried WHMCS. What a revelation.
Easy to install, easy to configure. Seems to have all the features I could want. I host my design and marketing clients rather than being an all out hosting provider, so there may be something I'm missing. But I don't see it.
I even had a support question shortly after installing. I put in a ticket and Matt responded within 10 minutes! I nearly fell out of my chair when the email came back so fast. Now I assume that most support issues take at least a little longer and I just happened to hit it right, but it's a good sign that somebody's on the ball here.
If you haven't tried it yet, I can recommend WHMCS. It's a great fit for anyone with a business model like mine and maybe a lot of others too. I do know I keep hearing good word of mouth about it around the web these days.