To be honest my biggest concern is the fact that Plesk 10 has so clearly removed old functionality.
An update is supposed to be adding features (or fixing issues). Not taking away what is already there, or transform itself into a totally different product, unexpectedly.
Good suggestion to acquire an extra server, just to test Plesk 10 functionality. Would make sense if it was a bug-testing session, but bug-testing is what I expect Parallels to do.. But one should not have to "test" for existing functionality having been thrown out.
What would you say if you had been using Microsoft's Office Outlook for many years, upgraded to get a new pretty interface and found out that it suddenly only supported connections to Exchange servers; that it no longer could download mail from your IMAP or POP accounts. You would call it insanity. Why remove features that are in use and are working fine. (Actually a good example, because Microsoft made that mistake in Office 2003, if you remember. They added new security that made it impossible to open many types of attachments. And was forced to make "fixes" that restored the old functionality, after their users revolted.)
With new Plesk, there seems to more of a existing functionality removal and functionality shift from Plesk 9 to Plesk 10, than there would be to port domains over to another product. At least one would know up front that this other product would be different and would expect it.
Plus as a many year developer I do not understand how someone can create a new structure, where I am forced to move from being able to control all my domains from one root screen to suddenly needing to jump in and out of a whole stack of separate "Control Panel" windows to see the same information old plesk could could show in a simple overview. Add to that longer sequences of clicks to open windows, and more clicks to open hosting/domain panels in those windows, plus cutting/pasting to put the information together. Thats really bad usability. I don't understand who approved that level of usability. Independent of it being a totally different functionality from older versions.
Its OK if Plesk wants to add more functionality to support people selling Hosting. Thats their business decision. But that can be done by adding features. (Plus, from what I have seen of comments from people using it as a Hosting controller, they lost out and are even losing their own customers as well.)
You don't have to alienate and destroy your old customers on the way to acquiring new ones. That is bad business. Acquiring new customers is nice, but repeat business and existing customers renewing their licenses is what pays the bills.