OK, let me try to answer your questions in order here. The card works without any 3rd party drivers. Plug it in and OSX sees it as a native firewire bus, almost. Before I got my hands on a FireWire burner, I wasn't sure if the card would work or not. OSX gave no indication it even knew the card was there. System profiler didn't list it or anything. So I guess that answers your question about the card showing up on the desktop. As far as I know OSX currently doesn't deal with PC cards all that well. No hot swapping, they don't show up on the desktop, etc. Luckily all that's supposed to change with 10.1. It's not a big problem either way...I just plugged the firewire burner in and OSX saw it just fine.
As for usability on my machine....I've been using it as my primary OS since March. Granted, it runs kind of slow. Apps take a while to open, the Finder is very slow, menu access is pretty slow. It's a little irritating, but bearable. My friend's got an iMac 450 (I think) and the only place I notice a difference is in 3D apps. I can't even use the built in screen saver because it's too slow (2-3 frames per second). Currently OSX doesn't really support our video cards (Rage Pro). The lowest end video card fairly well supported right now is the Rage 128. I haven't heard a definitive answer yet about whether or not 10.1 will fix this oversight. I'm keeping my fingers crossed.
I've heard from one person with a Wallstreet powerbook running some beta releases of 10.1 on his book, and he's very happy with the speed on it, so we may be in luck. 3D graphics on it still sucked, but with any luck the pre-release versions he's been running just don't have good drivers for the Rage Pro yet and the final code will.
Overall though, I've been very happy with it. It's solid as a rock and has a lot of potential. Hopefully by next week or so, 10.1 will be out and some of that potential will be filled.
I do know that when X first came out a lot of Lombard users had problems with 3rd party RAM in their machines. Bad CPU daughter card or something. Don't know if there was ever a resolution for it, but I think it only affects machines released earlier in the year. I got mine in Sept. 99 and haven't had a single problem with it. That may be something you'll want to take a look at though. You can probably find some of the old threads about it at macfixit.com
If I can answer any more of your questions, feel free to let me know.