A few mates and I have recently enrolled in a short Z Brush beginners course, running a few concecutive Saturdays and Victoria Uni in the city. We just recently did our second class which covered z spheres 1 and 2.
What an awesome program. Well equal parts awesome and equal parts frustrating i guess. I've never encountered a program that you can so easily completely lose your file if you're not sure what you are doing. That said though, I've also never encountered a program that has so many awesome features.
I can see I'm going to have a lot of fun with this program once I learn how to use it!!!