Pixologic specializes in 3D sculpting software, such as the free and easy-to-use Sculptris. ZBrush is best thought of as Sculptris's advanced cousin, as it's also focused on sculpting 3D figures, but at an exponentially greater level of control. There are a ton of recommendations on ZBrush's website from 3D animators from films such as Chronicles of Narnia: The Voyage of the Dawn Treader, and that's because professional tool ZBrush ($700, 30-day free trial) is very intuitive to use.
Despite being somewhat of a 3D sculpting newbie (though I did review Sculptris), I took to ZBrush easily. It's got a high learning curve, but it's user-friendly at the same time. I managed to take a rhino figure and twist and distort it and even give it a sphere body with ease. Several more hours with this product would give me a lot of practice making 3D figures.
And that's what ZBrush is useful for: designing individual characters in 3D as if you were modeling them out of clay, and also designing environments. The nice thing is, if you want that simplicity that sister product Sculptris provides, it's easy to go back and forth between the apps using the Preferences -> GoZ menu. Files created in ZBrush can also export into many other popular 3D graphics applications such as 3D Studio Max.
One thing I missed was standard Windows 7 menus; instead, the menus are built-in to the app and are in a weird order ("Document" was the fourth from the left, for example). You can't click one menu and page through the others with the left and right arrow keys like with a Win 7 menu, for example.
Although ZBrush is an expensive application, it's on par with other, similar apps, and would be a welcome arsenal in any serious 3D graphic artist's library. Though it's in competition with 3D graphics programs like 3D Studio Max, it's really complementary to those applications as they perform different functions in the 3D graphics space.
Note: The trial of ZBrush is full-featured for 30 days of actual running time, and you have 45 days to use it. However, when you close and re-open the trial, you'll have to activate it on the website with your registered username again, so you're better off leaving it open. The full version is $700.