Yes, AIX 4.1.5 with my own security patches. I shouldn't be too hard on it, it's a pretty busy server, considering (the web server handles around 10,000 transactions a day -- not bad for a vintage server).
Part of the problem is probably the RAM. It uses the parity FPM option, and when it heats up, a bit probably flips somewhere and AIX MCHKs since it doesn't do ECC. The system stays up, but it forces a reboot and kicks me off. This happens a couple times a week and it's enough to finally annoy me sufficiently. However, NSDU can't handle more than 256MB of RAM and the RAM always passes the ROM-based Long RAM test because nothing's hot yet, so I've never been able to track it down (and try finding parity FPM RAM today!). Cache had also occurred to me, but I would have expected more than just an intermittent fault for that, and it kernel-panics with a stock PCI Power Mac cache stick so I'd have to wait around for another 700 1MB cache.
By comparison, the Power Mac 7300 that runs my Gopher server is rock-solid, but there's no parity RAM (let alone ECC), and it's got a G3 card and I've got plenty of spares for that (I bought lots of Sonnet's CPU cards when they were blowing them out so I would have parts). I think the issues I'm seeing are more specific to the ANS, because after all it was intended as Apple's "big iron."
At one time trag was looking at designing a CPU card for the ANS, but he never was able to get far enough with it, and I think there would be exactly two people who would buy it (he and I). I'm sure there are people still using ANSes in production environments, but very few, and I'm the only one I know of that still runs AIX.