J!WHMCS is a very high quality and well planned Extension for Joomla! and WHMCS. The functionality is very reliable and user friendly. The installation is an automatic smooth process. It includes an automated update process which I think is key. It also handles the installation of the WHMCS side, considering you make a simple API connection first. I personally used Joomla! 3.0 and the latest version of WHMCS with no problems.
Best of all, the support that the Go Higher IS staff provides is the best I have ever seen. They respond promptly, professionally and with thorough detail. They go above and beyond the product to make sure it works with your template.