We have been on Clientexec for about 6 years now. Development is steady but slow since it is being done by a group of 3-4 folks. Not as many plugins as WHMCS but is a much more stable system. Less security holes and readily available support.
While WHMCS is kind of the industry standard, you must understand one thing. Their support is literally Zero. Unless you are willing to wait for upto 24hours or actually pay $$ for each ticket you raise, the support is sometimes non existent. Yes, there is a lot of documentation around all the issues with WHMCS however, you will still have problems unique to your setup which can only be resolved with support and that's the drawback for them I guess. Also, they have moved to a billing model based on number of clients instead of a standard charge.
Also, remember, WHMCS has the same problems windows has which is because its so widely used, the security exploits are so frequent and there has been quite a few incidents of poor security implementation causing huge losses to few hosters. So, consider all the issues holistically before making your decision.
Having said that, the number of features and plugins WHMCS has is yet to be achieved by any competitor. A lot of themes that match your website are also available for WHMCS and a lot of the stuff is modifiable. If you wanted another provider to develop a plugin for you, they would easily do it for WHMCS but they will hesitate for any other billing application
