Even a PS2 KB is fine with a Mac with a USB/PS2 adapter. I waver between two different PS/2 KBs on my Mac Pro at work-one a pretty standard c. 1991 IBM Model M, and the other a Unicomp Model M from 2002.
With Basketball season building, it's about time to get my Unicomp back out, although I usually save it for March 🙂 . My Unicomp is the University of Kentucky special edition(I work at arch rival University of Louisville).
BTW, I LOVE the folks at Unicomp. When I picked up my UK Model M at a flea market(for $5!) I shot off an email to the factory to ask about the missing blue "Alt" key. They found one for me, and swapped it out while I waited when I look the board into the factory. I've been putting off buying a USB Spacesaver M, although it's on the agenda.
I like the original Apple Extended Keyboards also, although I'm constantly rebuilding key switches in them(I haven't had to do that on a model M).