Can resellers and subresellers create their own branding as well?
Yes, all your resellers and subresellers can all have their own branding.
Can you explain a bit about user management?
For example, is it possible to switch users, using something like a single-sign-on? For instance, assuming you are a reseller, can you login as a reseller? But can you also prevent resellers to login as users from subresellers? This might be feasible for security purposes.
PowerPanel supports three roles: admin, reseller or customer. You decide whether or not a customer has permissions to a certain tab in PowerPanel. A customer may only have permission to the hosting tab, for example, allowing him to edit his website. A subreseller on the other hand has rights to the invoices tab.
With Plesk you can make Reseller Plans, and I would like to charge my resellers only for the disk space and bandwidth that he actually used. Is this possible? Or can I only charge them a fixed fee? I ask this question because WHMCS can only bill “Customers” for the usage data, and it won't update the usage on “Resellers”. I would love this in PowerPanel.
It is possible to charge your Resellers for the bandwidth and disk space they have used, but it isn't natively build in. With PowerPanel you can charge overuse your resellers if they use more than the hosting plan you have created. It should be possible to create a hosting plan with 0 bandwith or disk space and then charge your resellers for the overuse. This function has not been tested yet.
How does the reseller option work?
Technically speaking you are a reseller from the PowerPanel perspective. So a reseller works just like your own account if you become a reseller of PowerPanel. Only it functions under your own account, your reseller can have reseller and customers on his own.