Never used HostBill & never will. Far too expensive & lacks tons of stuff...
The admin interface is extremely weird, hard to locate things & is complicated & the theme is completely nasty. However, I have known customers to use it without issue - my main issue is support for the software after the first year, which I find ridiculous ($50/ticket), if you have a software, you should support all your customers in my opinion, not charge them for support. Which is why I like WHMCS, they provide support & updates for a fixed annual price (I have an owned license, just fyi), which comes with unlimited tickets & updates within that time frame, which is acceptable, as I would expect to contribute to the further development of the software, but not on a per ticket basis.