In a virtual machine, mostly. I have a few open source works in progress that I want to work on everything, Haiku included (and most likely Windows excluded because it just has to be the special snowflake of OSes and I'm not rewriting half of my programs just for it).
I have played around with Haiku on real hardware though, running a full install from USB on my laptop (which worked surprisingly well). I'd probably use it more if I had an old Thinkpad or something to spare.
The OS has a long way to go before it can replace macOS or Linux on my machine, but it's interesting. WebPositive works pretty well but Safari still wins by a mile so my browsing still takes place in macOS. And I haven't gotten around to see if Python 3 works on Haiku so I still do that in macOS or Linux