I've been a BeOS user since 1998, I think (when R3 came out for x86).
I can't really say since when I'm running Haiku natively, it must be around 7 or 8 years, maybe more...
There's Linux (Ubuntu? or maybe Mint?) on a dual-boot, but I only boot into it every other week for some LibreOffice etc. for some years now.
I don't think there's a "killer app" for Haiku, unless you go into an extreme niche like the radio broadcasting app of TuneTracker.
To me, Haiku itself is the killer app. Being a complete OS, we have the opportunity to seamlessly integrate every part of the system. Sadly we're limited by the low number of active developers...
Of course, I'm biased through almost decades of using BeOS/Haiku, but I continually miss features when I have to use other OS, e.g.:
- moving/resizing windows with CTRL+ALT+mouse,
- right-click to send a window to the back,
- easily moving windows across different workspaces,
- a nice and lasting GUI
- Tracker's type-ahead filtering,
- consistent drag&drop into file panels (in other OS, I never know will it move the file or change into its folder),
- Tracker add-ons
- fast file queries
- sane folder hierarchy
- being single-user (i.e. don't bother me with permission alerts)
All that said, I do recognize that my main computing is for the Haiku project itself. A bit of coding, a bit of documenting and translating, connecting to the nicely small community...
Haiku is ready to be used as main system, but only if your interests align. It's not for 3D artists or video editors or 3D gamers, but if you're interested in computing for computing's sake, like the early days of Amiga et al, Haiku is here for you.
Of course, as the software environment grows, more uses become feasible. It all depends. Pete is doing OK with making music right now, others need way more feature-rich or easier to use apps for that. Pete is a coding musician, so not exactly mainstream. OTOH as we've seen with Be Inc.'s Cotton Squares, it doesn't seem too small a niche...
That's just my personal opinion, of course. There are probably as many as users of the system (and possibly twice as many from people that haven't used it...)