Way back in 2006 I attempted to use a Pentium II machine as a daily machine. I thought it would be a neat little experiment. I lasted about 2 weeks. Windows 98 was just driving me bonkers.
These days, my use of older computers is limited to Win9x/DOS/console/home-computer gaming and experimentation. I don't even have a dedicated XP/DX9 gaming rig anymore (just one year ago, I had two machines dedicated to running XP era games!) Turns out that my 4.5GHz 4930K running Win7 is more compatible with DX9 and even DX8 games than I had originally thought! And it loads and runs those games so freakin fast that I can't imagine going back to anything slower.
Don't get me wrong--I still love collecting older hardware. Whenever I see anything remotely interesting being thrown away, I take it home and usually keep it. Experimenting with older gear and pushing it to the limit is always very fun. My collection of old hardware and software is bigger than it's ever been and I will continue growing it. I usually fire up my 98/DOS machine (Tualeron-1400, V3-3000, two sound cards--the whole shebang) once a week to engage in some serious old-school gaming. It's even connected to the internet! I'm not too concerned about malware--none of that stuff affects Win98.
But my computers that see daily use are always going to be modern systems. Using a Pentium II as a main PC was an enormous pain in the ass back in 2006; I doubt it would be any better today.
94 MHz NEC VR4300 | SGI Reality CoPro | 8MB RDRAM | Each game gets its own SSD - nooice!