VMS is a very stable operating system. It's not Unix neither Mainframe, but its market is roughly between those two - high end servers, workstations, and distributed systems with very high security demands (C2 out of the box and with a good system manager it can go higher). Security features include (optional) auditing of virtually all system components, fine-grained access control lists and user privileges, detailed control of every user's console and graphics shell settings. It supports X-Windows, Motif and CDE; as for networking - it has it's own proprietary DECnet protocol, but lately TCP/IP is being integrated more and more and it's possible (and quite easy) to setup a web/ftp/telnet/mailserver on VMS.
Although it has a reputation of being slow and old-fashioned and is usually connected to the old VAX computers, that is wrong - the system is being constantly updated; Compaq's recently updated OpenVMS roadmap is showing that they aim to make the system more open, make more (and by mid-2001 eventually all) system parameters (such as MAXPROCESSCNT) dynamic, so one doesn't have to reboot to activate the changes. It's a pretty easy system for the manager, however its philosophy is somewhat different from Unix in this area: there's only one shell - DCL, which is quite primitive as a language - IF and GOTO are the only control structures, there are virtually no data structures, etc. - but a lot can be done using it's so-called "lexcial functions" (essentially system calls). Though AFAIK versions of bash, csh and the like exist, they are not widely used. Unlike Unix, where the manager often edits various text files to change the config, the management in VMS is done mostly through commands and "command procedures" (scripts) - most of configuration files are binary, unlike Unix, for instance SYSUAF.DAT (the VMS variant of Unix's passwd).
A lot of free software, including EMACS, Perl, Python, Apache, gcc, Mozilla and xbill was ported to VMS. For programmers - it supports POSIX, modern memory management (it was one of the pioneers in the field of paging), threading.
To conclude - it's not Unix, and it's stinking proprietary, but it's good if you have a lot of money and need very high security and scalability.