Until last weekend, the Mac mini bringing you these words had been running NetBSD 2.0.3, because 3.x had been unstable on it. After making careful worst-case preparations, I bit the bullet and tried upgrading to 4.0.
To prepare, I set up a spare machine. Since the instability with 3.x appeared limited to my model of Mac mini, I felt safe choosing a dual G4 as the spare. On it, I installed NetBSD 4.0 as though fully intending for it become my new server: assigned it a static IP address, merged /etc, synced entire databases and home directories, and built the roughly 300 packages. If everything went well upgrading the mini, the pre-built packages would minimize downtime to web and mail service; if something went wrong, the dual G4 would be ready to go.
Well, nothing went wrong. NetBSD 4.0 has been completely solid on the mini (and the dual G4, while I needed it). It's nice to be running the latest and greatest (major changes in 3.0 and 4.0) for the usual reasons, plus a very practical one: the improved FireWire device support should let me use the stackable external hard drive (bundled with the mini when I bought it on eBay) as a full, bootable backup.