I want to say only one thing.
I think that to manage shared hosting servers using plesk or another control panel, needs to be done by IIS trained IT pepople.
In our company, we are using plesk for windows since first version (6.5) and, we've only called support one or two times and it was for things that couldn't be repaired as IT people.
(I remember one for the mailenable logging that it wasn't rotating logs, and another like that). Our problems were solved in the next release of plesk
The problems like passwords prompt and more are typical IIS errors / misconfiguration problems that a person trained in IIS can resolve in less than 15 minutes. (Like integrated authentication that doesn't work with proxies or IIS password sync that can be solved reaplying the password to the user).
If you don't know how to use IIS or if you know how to do it, you don't want to do it, I'll say that there is no perfect CP, becuase there are more thing in the IIS metabase than some permisions, some syncronized passwords and so. It is a really huge file and is hard to control with some PHP's and some exe's.
I like helping people, but the only thing I want to say is that to manage a server farm, you should be a qualyfied technician (Or you should look to take one). This is not like using your IIS in windows XP. Also think that you shouldn't have more than 100 - 300 domains in one plesk.
Whith more domains, it becomes more hard to manage and every server becomes more important.
The only thing I can say here is that in these years, swsoft has been releasing better products (The new versions always are like beta versions, I know) but plesk everyday is doing more and more things and all tehir products everyday has more features.
Regards.
Arnau Marcé