I use Haiku on an old Pentium III because Linux (with graphics) bogs down too much on it (unless I use an extremely old version of Linux).
I also use it on a more recent AMD 64 machine (3 GHz, N-68 ASUS motherboard). It works well on the AMD machine, which normally runs FreeBSD. What's nice about Haiku is that I can boot it via a CD or USB image very snappily for a quick internet session, and not expose (very much) my underlying HD based OS.
Web+ can try the edges of my patience level on some sites, but it does just fine on forums. So, typically I'll quick-boot it for a peek at a forum. Some time ago I moved the machine to a place where the ethernet cable doesn't reach. So, I was not on Haiku for a while. Finally, I bought an old NetGear, Atheros chipset based PCI wireless WiFi card (WPN-311, circa 1999) - for pretty cheap, and so can use Haiku again from the remote machine (what I'm using right now).
Have used JOSM (open street maps) on Haiku, and Scribbus. It's also a handy tool for putting a new partition table on a drive, and for creating partitions (which I may later use for non-Haiku purposes). So, Haiku has a few niche uses for me (and internet almost every day).