Absolutely not user-friendly
Perhaps you can work with this software if you grew up with it, but the interface is probably worse than Blender (a free 3D-program). I hope to learn how to use it by watching enough videos from digital-tutors. The so called ZClassroom offers poor tutorials.
For now I am puzzled and dazzled by this software. Perhaps when I have mastered the interface, I will be able to write a positive review.
Further more: it is not a stable program. Just now it crashed again. I have the latest version 4R4 P2, which took me some time to install. In most of the software-packages it is common to have an auto update facility, but in ZBrush it is a plug-in. This plug-in did not work in Mountain Lion because of a security-issue. I had to look for a separate app "ZBrush updater" to install the latest version.