I started out around 3 years ago with CE had nothing but problems with it. Then I switched to MB (don't attempt to use v5) this software was ok, but is hard to incorporate custom modules,billing and plans. Then I decided to switch my billing from prorated to anniversary billing (big mistake) lost all of my monthly billing so I had to manually run cron jobs every month. Anyways their support was about as low as a curb on the second hand streets in town. To make a long story short after 3 billing software migrations I've had nothing but compliments from my clients about WHMCS.