Unless you feel like playing with an old and obscure OS, BeOS really isn't worth learning. It never was very popular, there is very little commercial software available for it, and the company that created it is now out of business. There is a small hobbyist group that continues to support it, but it's probably safe to say that if won't be making a major comeback.
You would be better off learning UNIX, or Windows .NET Server. Personally, I'd give Linux or FreeBSD a try, because you can use things that you'll learn from those OS'es in almost UNIX-like operating system.