You guys will love version 2 if you're only using the 1.55 demo. There were huge improvements to the tools and performance made. The 2.2 update which was just announced promises further improvements to the Zsphere tools which are an excellent way to approach character creation.
The interface isn't bad... just different. It's very focused on keeping you working in the viewport directly with your meshes as much as possible (with a tablet). I think a lot of new users find this daunting at first especially since it's NOT photoshop, nor does it work the way your favorite 3d app works.
The biggest deficiency for game character work in ZB lies in the fact that it doesn't have adequate UV tools. It uses a couple of auto unwrap procedures instead which are quite messy unless you plan on only painting your textures in 3d and don't care about uv layouts. The second biggest deficiency is that there are no proper polygon editing tools for maintaining absolute control over edgeflows and such. You're better off editing animation meshes in a real 3d app and using ZBrush for detail work or base mesh creation with Zspheres.
This link to the 2.2 update announcement:
promises to address some of the lack of polygon tools in a very unique, ZB fashion.
I currently use ZBrush for some character concepting in 3d (yeah it's fast enough to allow that). I generally start all my characters in ZB2 now using Zspheres, then export to a 3d app to fine tune the mesh, and uv map it. If I need high rez detail I'll send a copy of the final mesh back to ZB.
I'm hoping to be able to spend more time working on my animation meshes in ZB2 with this new update coming, as it seems to provide ways of generating lower resolution geometry with finer control - at least that's what the preview movies imply.
Check out another interesting use of Zbrush by Taron, a famous LW/Messiah artist and recent Zbrush convert:
Notice the deformation in the neck area. This is all achieved through Zbrush displacement maps driven by the control rig.
If you do character work and can afford it I'd say add it to your toolset. You won't regret it. Being able to work in 3d without polygon count nor edge flow constraints is very liberating, and allows you to focus more on the creative process rather than getting bogged down in the technicalities of character creation in 3d. Plus it's fast as hell. There is no faster way to generate high resolution characters in 3d by hand.