I would imagine that a lot of people who use OpenBSD on their laptops/desktops run a lean installation with one of the window managers in base (an ancient fvwm version, cwm which I find very nice and twm).

You can however have a full-fat desktop environment with xfce4 or gnome and applications like libreoffice, gimp, inkscape, audacity and so on if you wish. I've never tried KDE on top of OpenBSD base but I gather packages are in ports.

I think it is fair to say that the amd64 arch has good support. The i386 platform arch is on a 'best effort' basis these days which is understandable. I've never looked at the others.

SPARC is well supported (mostly because it's very good at finding bugs that wouldn't be big problems anywhere else despite not being 'correct') and big endian PowerPC (both 323 and 64) is fine, though hardware can be tricky since apple products tend to be so integrated that you can't really, say, replace a GPU because the support is poor