We custom built our own control panel in ASP.NET. We believe that it is better to build your own as a demonstration of your knowledge of technology, instead of using a canned hosting automation system. That is, of course, if you are a technology-centric host. For example, we specialize in ASP.NET Hosting. We also make sure that our website is programmed in ASP.NET as well.
Also, having a custom built control panel and hosting automation system powered by ASP.NET means that we can move quickly to offer new .NET technologies as they are released. Rather than, waiting for the third party vendor to do product development.
If you are not a technology-centric host (eg your focus is SME, designers..etc), then I would say using a third party control panel is fine - there are many to choose from: ensim, plesk, hsphere, cpanel - as mentioned in this thread. It will reduce costs and you can probably find support staff that already knows the control panel.
So what it boils down to is that it depends on who your target market is and if they will find a custom built control panel a value or not.
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