ZBrush has a "no time limit" trial version, but I found it hard to learn with that, since you can't save and reload your 3D models. Still, the free version is a great way to check out the interface and the many tutorials and to get a feel for the program.

Creative Power:
If your purpose is to make more or less complex 3D models for use in Atmo, ZB has the power you want. It seems to have most of what the high end packages do and some say it has even more power in certain areas. ZB is only in version 1.5, but already has most everything except nurbs, bones and bone animation. ZB does have animation (which I have not yet explored), but from what I gather from their forum, it is currently little used. More animation power is expected in the next release. The lead developer/inventor of ZB has released a video clip of an animated dinosaur he made that is very impressive, so strong animation is certainly coming.

For static models, ZB contains everything you need to sculpt, skin (make meshes from your sculpture), paint, UV map, texture, apply materials, light and export complex models. It's native 3D format is Lightwave *.obj, so models are ready to export for final processing in Viewpoint Scene Builder and then import into Atmo.

The only thing I found lacking is the ability to apply different types of UV maps to different parts of your model. But ZB works well with the free UV Mapper program. And ZB includes tutorials about using that excellent utility, if you model is that complex. For many models, ZB's built-in UV mapping is sufficient.

A look at the ZB forum will show you that ZB is certainly capable of fully professional static modeling. In many cases, of mind boggling quality!

Integration:
ZBrush works directly with *obj model files and *.psd textures (Photoshop native format) as well as other formats. So it is a natural for use with Atmo and Photoshop. ZB started as a 2.5D (paint+depth) painting program, so you can also do all your painting right in ZB, if you don't have Photoshop or just like to stay in one program.

Ease of use:
Is ZB easy? No, it is not. Which 3D solid modeler is?
However, it is a lot easier than other 3D solid Modeling software I've tried to use. For example, in less time than it took me to become discouraged (for the 3rd time) with trying to learn trueSpace, I had already produced 2 decent models in ZBrush.

ZB is designed expressly to be accessible to artists, rather than engineers. And I found the powerful 3D sculpting tools are pretty easy, once you understand their concepts. To my surprise, doing the painting, UV mapping and texturing was much harder than sculpting - the opposite of what I had expected! But I was still able to manage decent texturing, after some study of the docs and a couple of question in the helpful forum.

I believe that all the tools to facilitate standard industry methods for Atmo are there and each is easy and powerful. It's just that you need to understand quite a lot about how industry standard skinning, UV mapping, texturing, materials and meshes are supposed to work, to use these tools effectively.

It reminds me of learning Adobe Photoshop - a big set of powerful tools, yet with no hint about when, why or how to use each tool. Unlike Photoshop, ZB has powerful ZScripts. These are built-in tutorials that record and recreate actual annotated modeling sessions, to train you in the interface, tools, tips and tricks. You can also record your own ZScripts to automate tasks, or to demo your work flow to others.

Happily, sculpting in ZB requires much less technical know-how than does mapping and texturing, so creating models is quite straight forward and powerful.

The Community:
ZBrush has a very active, friendly and enthusiastic forum and user community. The quality, speed and accuracy of the help is just outstanding! And every day brings several works-in-progress to the forum, where you can follow their daily progress and work flow. Often new ZScripts or 2D graphics are included, to show you exactly how the more advanced users do their magic.

Interestingly, many of the advanced fans of ZB also work in the high end professional packages and move freely back and forth between them and ZBrush. They use ZB to do things they feel are much easier or that allow them more creative freedom than in those expensive pro packages.