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I've been running OS X exclusively for over a year and haven't had any significant problems. It may be well be the most crashiest Unix that I've used in that I have had to reboot it every once in a while--but "crashiest Unix" is pretty relative. There's nothing I've seen that makes it slow down over time as you described, although that's behavior I've certainly seen under Windows 2000 and previous. I haven't used XP enough to speak to it. My biggest problem with Windows in all the versions I've used (3.1, 95, NT 3.51, NT 4 and 2000) has been what I've dubbed "creeping crud syndrome": over time, during perfectly normal use (even somewhat conservative use), the system slowly becomes less stable. Invariably, 18-24 months after the initial install, I find myself doing a complete reinstall. (That's how I became a full-time BeOS user for a while.) For all the Mac's foibles, I haven't encountered this on any Macintosh operating system, and I've used System 6, System 7, MacOS 9.0-9.2 and OS X 10.1-10.2. (Pre-X versions of Mac OS could be made really unstable through extensions, but removing them was--usually!--pretty simple.)
I doubt there's significant differences in plug-and-play anymore. To me networking on the Mac is a bit easier to set up, but it's marginal (and I've watched a very Windows-savvy friend be utterly confused by the Mac way of doing things, so I'm aware it's partially learned behavior).