Is there a live/eval version of BeOS that can be downloaded and burned under Linux? Is BeOS fairly easy to use, with a nce gui/command line combination?

Yes there is a LiveCD Distro of BeOS.
http://www.bebits.com/app/3861

And here is an instruction to burn the .cue and .img files.
http://members.xoom.virgilio.it/motoyama/

To answer the other questions...

BeOS is very easy to use. It was designed by a few former Apple empoyees including Be Inc.'s CEO Jean-Louis Gassée who was

BeOS is not based on UNIX (however it is POSIX complient), a common misconception due to Be's inclusion of the Bash command shell and some similarities in its directory structure. However if you are a programmer, you will find the C++ API enviroment to be extreamly easy to program in.

I would suggest you install a full developer version (free) from:
http://www.beosonline.com/
It's a bit strange to burn to CD (not an .iso), but I don't think it should be much of a problem if you can follow simple instruction.

The real reason why I would rather see you install BeOS full version is because to understand how fast BeOS is, you need to see the speed in the filesystem. It has an optimised 64-bit journaling and indexed file system called BFS, but rather than use a database BeOS relied on its low OS latencies to journal and query file attributes on the fly.