Do you ever want to try an old machine, just because you're curious as to how it works? It's happened many times for me, and in all those times, I give up on the 3rd day. Why, you ask? Because it's basically useless today! Old browsers don't support Flash, Silverlight (article coming soon), etc. And most old computers don't have ethernet (hello, dial-up!). I still wonder how dial-up internet companies still make money today.
The biggest reasons as to not buy an old Apple computer is because that nothing's really compatible with it anymore. Apple's move to Unix in OS X was great, but it left many classic apps behind. So if you're going to buy an old Mac, it'll be hard to find software for it, unless it's a System 7 machine. System 7 Today is a great website for info on integrating System 7 into your daily life.
The same goes for most old PC's, although you might be able to find some programs that still run in Windows 98. Forget 95 and earlier. Microsoft has left DOS as an integral part of the modern Windows OS, so if you have one of those old DOS games on a Twiggy, you can probably fire them up in XP or Vista.
I have moved my blog to Wordpress at theunixgeek.wordpress.com. I will still be checking back periodically on this one as well, though. 19 April 2009