I can say that Webmin, and yes, I know, that wasn't in the original post... But, Webmin is very good about first not taking over anything and it is also pretty darned happy about admins working directly from the command line or from within the GUI. It is however a bit of a geeky interface. In many intances the interface provides direct access to the config files as an alternative. Using Webmin has saved me countless hours of time. It takes a lot of time to set up the module configs so that things are done the way you want to do thing and one needs to be careful to understand what gets stored within Webmin with regards to users, vhosts, etc. as somethings sort of do need to be done from webmin or you'll not have access to deal with them from webmin later.
I have no direct experience with cpanel or plesk, but was a bit involved with a plesk machine from the user side and found it to be very frustrating. There was a lot you simply couldn't do from the interface that was provided in that situation, but maybe it was there to turn on if the sysadmins had set it up on the other side?
Again though, Webmin is pretty geeky and really is a tool for existing admins with knowledge, not a tool to replace that knowledge. It is a timesave as it is faster at things like adding a new virtual hosting account.. once setup, just a few entries in a form and absolutely everything is done for you, from bind through email. No way I can be that fast via the command line.
Webmin scares me more from a security perspective (as do the others) mostly because I don't like the idea of a "one interface to rule them all" type of approach. There have been several privilege escalation type holes found in webmin, and if someone gets access to it they can do whatever they want to your system. I suppose the same is true for cpanel and plesk, although I dislike them for other reasons. Or it could be the fact that I'm firmly entrenched in the "Commandline Admin" camp with no intention of moving. I've seen far too many "Admins" click a machine to death with lack of understanding. In fairness, they do this on the command line also with --force or --nodeps etc.. but in smaller numbers.
-- "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety'' Benjamin Franklin 1775