Hmmmm, why to switch, the question of the decade
I recently switched for a variety of reasons. I've been a Unix person for years and years who has been forced to also have Windows around for those apps that don't exist in the Unix world. Well, all that has changed. I can now do everything I need to do, and did in Unix and Windows in OS X. I've found the productivity apps much more stable on OS X then in the windows world (Office, Acrobat, Quicken) and they are much more intuative IMHO. The biggest things that are lacking is some of the multimedia stuff (which surprised me). Alot of codecs aren't there for quicktime, and no real support for 5.1 digital audio (atleast that a consumer can afford).
As to the cost of upgrading, in honesty it didn't really cost me much at all. By selling off all my old hardware (SGI's, Sun's, PC's, etc) it just about covered the cost of my mac, and having just gotten my first power bill since turning off my old computers, I'm saving around $1.50-2.00/day on energy costs. So, thats $550-$700 or so a year I'm saving.
But ofcourse, everything is based on the user and what their needs are. I'd suggest picking up a used G4 sawtooth box, and play with it and see if it does what you need. If it doesn't, you can turn around and sell it again with just about no, if any at all, loss in value (assuming you figure out in a month or so if you like it or not). If you do like it, you can either keep the used box you bought, or sell it and buy something new and speedy. Either way, you really don't have anything to lose going that way.
UNIX is simple and coherent, but it takes a true genius (or a programmer at any rate) to understand and appreciate its simplicity -- Dennis Ritchie