Me too. WHMCS is very professional, but it doesn't mean HostBill isn't reliable. In addition you can do some customization work with HosBill according your needs, but you cannot modify code of WHMCS, as it's an encrypted. And for more information you should consider below URL. Hope that be helpful for you.
To be very honest, we are using both WHMCS and HostBill. However I find OnApp is far more suitable with HostBill. Anyway, you will have to pay additional charges for OnApp plug-ins to get all the features with HostBill.
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+1 WHMCS. It is a great software for any environment. It is great in functionality and like a 100% bug free software. In short, a complete software for web hosting business.
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