Funny thing is that in a ( very ) older version my engine compiled and ran on BeOS ( back then one of those modified personal versions... don't think it's still up on the net though ). Played Quake2 and Quake on that same BeOS.
It's unfair to be niche though. It's faster than Linux ( and therefore way faster than Windows since Windows is a lot slower than Linux )
and the DB file system is just sweet. Why the hell did Linux not get such a splendid thing? :/