Hi, just want to share my experience with WHMCS billing system.
Im using it almost 5 months. Here is what are my thoughts.

- I think WHMCS is maybe the best turnkey billing system for webhosting reselling. Because its all containing interface. many hosting clients used it, so they can manage it easilly. Also its the best because it containing addons of tens of payment gateways like paypal, credit card, google checkout and much more.
WHMCS account autoprovisioning module supports also cPanel, LXAdmin (kloxo), HyperV, HyperVM, SolusVM and more VPS reseller systems or Hosting reseller systems. So after paypal payment, VPS or hosting can be setup/suspended/terminated usually automatically.

What i dislike:
- Learning of WHMCS took me around 2 months at least! And during my 5 months time, i had to submit around 30 tickets / 60 replies to WHMCS support. So it is not easiest to manage and modiffy everything.

What i like:
- WHMCS support is quite fast (reply usually within 1-18 hours and support is helpfull. They are doing good job.
- very helpfull knowledgebase with tutorials
- very wide customer base makes it easy to find solutions and addons on Google

Very important tip for WHMCS owner!
DOnt underestimate importante of daily whmcs mysql backups!!!!! When you loose your billing history continuity, it would take HUGE amount of time to add all these transactions manually. Save yourself a time to do daily whmcs mysql backups. Shedule it or ask whmcs to make automation backup solution.