Netware 6 has so many nice new features / advantages that it's worth the effort to upgrade.
I've also heard of a number of NT4 / win2K servers (where the company wasn't ready for Linux, and was tired of weekly or daily reboots and virus/worm attacks) moving to NetWare 6.
The new licensing by user - with essentially unlimited servers will mean a cost savings in most multi server installations.
NetWare 6 has "clientless" access from Linux using nfs. Mac's using appletalk. and (I'll say it quietly) those machines with evil software from Redmond using CIFS.
also of interest for those running window$
iFolder - think of an internet based "briefcase" on steroids that automatically keeps a sync copy of your files on the server.
iPrint - printing using internet protocols with browser based printer picking and automatic driver download.
Novell is supporting many open development projects like openldap.org and their announced development direction is to be "closer" to the open source community. Perl, , tomcat, apache are now part of the standard install, and I've heard that ports of python and php are either in the works or released. Java is also standard. The internal architecture changes in 6 and more changes in upcoming version(s) are supposed to make "porting" open source projects much easier.
Announced futures directions include a VM type of architecture where a NetWare kernel could run next to a Linux kernel bringing the strengths of both to the system.
Now if only they could figure out how to market around the redmond juggernaut.