I use Mac OS X right now and have yet to use any *nix other than it (a while ago we considered getting a Linux PC but after much research on my part decided to get another OS X machine- an iBook- and keep an eye on Linux), although I use a good bit of X11 and CLI software with it. If I couldn't use Mac OS X, I'd use Linux or FreeBSD/Darwin. A case in point is PDAs, which Mac OS X can not run on. I plan on getting a Sharp Zaurus, a Linux-based PDA, this summer. If I couldn't use Linux on the PDA, I'd go with PocketPC or a Newton.