I love OS X. I use it at home and at work (when possible). I'm in IT (~100 employee publishing house), and here's my story...
We've got approximately 100 machines, the most recent of which are G4 400's. The vast majority of our machines are rev. D iMacs (333). It's pretty much just me running the show here as far as desktop support and maintenance goes, so naturally not all of the machines are as optimized as they could be (read: they're sometimes painfully slow and unresponsive even under OS9). In this economy, we're not likely to do much upgrading of hardware anytime soon. In this regard, Apple has almost painted itself into a corner by producing quality hardware and software (relative to Wintel) such that we're in a if-it's-not-broke-don't-pay-to-upgrade-it cycle.
I can't imagine that we're alone in this - we at least have some fruit-flavored machines. I know personally of many, many shops in this city that are all beige, all the time. Like another poster said, there's still a lot of pre-G3 out there.
It's a hard, hard sell in this economy to put a new OS (and new apps) on the desktops when there isn't a severe performance benefit visible. And for the majority of our clients running Office, web, and email - OS X doesn't offer this great benefit. For me, doing more networky stuff, it does, so I use it.
When Jag's released, I will be thoroughly testing it on our rev. D's. I hope against hope that it will be as fast (or nearly so) as OS 9. If so, only then can I start to push for it's adoption here and start selling its other good features without being stopped cold at the "god, it's slow" first impression. I would love to have a more stable, simple, and easier to maintain OS for some of our less computer-savvy clients, and I think OS X could be it.
But these are only some of the problems and associatied costs. The ridiculous price of Office is a problem. We'll have to upgrade Retrospect, FileMaker, and a pile of other software (using Classic or a dual-boot system is something I am NOT prepared to do with our clients). And it's not really ready for adoption by our production staff, for sure. Quark? Font management? Printing issues? We'll have to contend with all of this...
Gah...now I've got a headache... :)