I haven't had that much time to tinker yet, so take this with a grain of salt. I am running Mandrake Linux 8.2, Windows, and OpenBSD here, and I manage about a hundred Debian Linux boxes at work from my Mandrake workstation. (I don't run Debian on any desktops, so I will limit my comments to the Mandrake distribution.) I have a pretty wide exposure to a lot of different things. The G4 is a new addition, but I'm not totally ignorant on the BSD underpinnings.
My impression is that the G4 is really gorgeous, but not very well-integrated into its Unix guts. Some examples:
A) The Mac filesystem (HFS) isn't case sensitive, and this causes some very strange problems from the command line. Case-inensitivity is undoubtedly better from a people perspective (I remember what a hard time I with case issues coming to Unix from DOS), but trying to remove case sensitivity from Unix is a Herculean task, and probably impossible. You don't casually change an underlying assumption that has thirty years' worth of software written on top.
B) You can't, from what I can see, launch a command-line program from the Finder, and you can't launch a Carbon/Cocoa app from the command line. That's exceptionally poor integration. Even the 1985 Amiga did this better.
C) Getting external Unix programs to work well is a lot harder than it is on Unix. Vim is an example... I run that on all my servers. It runs the same way on every variant of Linux I've used in the last two years. But the Mac version doesn't recognize pageup/pagedown keys (possibly because the Terminal is remapping them, not sure), and seems to be stuck in some older emulation mode that I haven't figured out how to switch away from. It still works, but it is painful... it reminds me VERY much of the early Linux days. This sort of thing was extremely common way back when. (note that this is possibly an issue with Fink instead of with VIM itself; I haven't tried downloading and compiling the package myself.)
D) There are no Mac drivers for my Lexmark Z51. This is a fairly common printer and isn't that old. Z52s are supported, but Z51s are not. For an operating system that claims 'it just works', this was a major failure from my perspective. It was the ONLY piece of external hardware I wanted it to drive, and it failed utterly.
And the mid-level stuff is all wonky on the Mac. Instead of rc scripts, they use this strange utility called SystemStarter. SystemStarter isn't very well-documented, and I have had very intermittent results getting things to run. There's a file called /etc/hostconfig that will supposedly let you turn on additional services. It claims that there are graphical utilities to adjust it, but I couldn't find any. I'm wondering if those tools are only included with OS X Server. I didn't have much luck getting services to run by manually adjusting it... in a couple of cases I had to create duplicate startup scripts that ran the appropriate utilities without checking that file. Definitely a hack.
SystemStarter itself looks like a good idea, and it's probably more extensible than System V runlevels; it'll make it much easier to ship new custom services and expect them to work on every OS X installation in existence. That would be really hard to do with rc scripts or System V runlevels. Improved or not, I'm suspicious it has some bugs still. I haven't yet spent enough time with it to be sure.
It feels like maybe Apple is trying to lock down Unix and make it proprietary so that they can charge extra for some bits of it. That is a strategy doomed to backfire, I believe.
I think maybe people aren't giving Linux enough credit for how far it has come. Mandrake is an incredibly polished distribution. It's not as pretty as Cocoa; it's not even in the same league. KDE, after looking at Aqua, is ugly. But while the surface is rough, the stuff *underneath* is extremely polished and well-put-together. Everything *just works* from an administrative perspective. It took about three minutes to set up my Z51 on Mandrake. (For a long time, in fact, Mandrake had better Z51 drivers than the official Windows ones provided by Lexmark! ) Ssh-agent is all set up properly ahead of time so that all you have to do is type ssh-add ONCE instead of a zillion times. (I had done this manually myself a few releases back.. it's not that hard but it's nice to see it integrated right into the default setup.) They need work on their GUI, but the polish on their utility layer is impressive.
Maybe Linux is being fixed from the bottom up instead of the top down. The G4 is gorgeous and has a great undercarriage and comfortable seats, but the controls are rudimentary and sometimes just don't work at all. As long as you want to drive on Apple-approved streets, it's easy and beautiful, but beware venturing off the well-paved roads.
Mandrake appears ill-built by comparison, but the undercarriage is equally solid, the mid-level stuff is well thought-through, and the interface is usable. Using the car analogy, it's sorta dirty and ugly when you sit down, the seats don't quite fit and you have to spend awhile adjusting them.... but once you get used to it, the 8-cylinder engine with the 4 wheel drive will let you go ANYWHERE. You DON'T spend all your time fighting it, like you always did with the older versions of Linux. The Mandrake team is doing an amazing job.
What I would really like is something with Mandrake's polish underneath and Aqua on top of that. I imagine I'll get it in another couple of years.
Right now, from a functional perspective, I'd have to give Mandrake the nod at the moment. I'm sure both will steadily improve, but if you are primarily a Unix user, and don't have a huge raft of Mac apps you need to run, OS X isn't quite there yet. It's fun to play with, and it is just amazingly lovely, but it is way behind Mandrake in terms of actual Unix functionality. Tread carefully.