I've been using Haiku since around 2009, and while I've no experience with BeOS itself (I'm far too young), I adore it. It's such a well thought out and designed system, right down to the APIs. Heck at this point, I'm tempted to use it for all my dev work (server side), as I enjoy using it so much. It lacks the quirks linux has across distros (somewhat an unfair comparison, as haiku has one distro and that's it), while being super quick and a joy to use. It's truly a desktop operating system, which I think is a good thing. It's like an unpolished, open source Mac OS in that way. The multicore and scheduler work being done is going to be handy for taking advantage of new computers.
Now building a desktop OS is a an odd niche. At this point, all of us live on the web, and the desktop matters little, as we have the move towards mobile OSes, and the integration of mobile features into our operating systems. If we ever see a move back towards more "traditional" desktop OS paradigms, perhaps Haiku will see more use, but for now it's community is small, passionate, and fun. I don't think it will ever be big, but one can dream right? :)