In the past couple of years I have been fortunate enough to work with a number of Unix variants: BSDI, FreeBSD, Linux, Solaris, HP/UX, IRIX, and A/IX.
Aside from learning what a *real* server OS feels like, I have learned that the dreaded "it makes you learn a bunch of cryptic commands" factor is a bunch of hooey. It's actually a lot simpler than other OS's because almost everything is a text file, or a file compiled from a text file. There's just not a lot of complexity - learn how to use any text editor and one command shell and you're done learning the interface. Or, use one of the GUI interfaces, of which there are several. I used to prefer AfterStep. Now I like KDE.
We have been very successful at setting up FreeBSD and Solaris servers for hosting, with few security or stability problems. I upgraded a Solaris 2.5.1 machine to 2.6, putting 2.6 on a new 9GB drive, and we added 256MB of RAM, without a problem. (The hardware upgrade happened 1 week before the OS upgrade; I'm not insane!) The whole OS upgrade took 5 hours and everything worked afterwards. This was the first Unix installation I had ever done. That is better than I can say for Windows - generally you have to reinstall EVERYTHING when you move everything to a new hard disk, because of that wonderful concept called the Registry.
Of course after a year of piling dozens of new client sites onto a machine it has to be upgraded (more RAM or disk space, usually, so that the machine will remain at .1% load level most of the time), and after a while there are lots of old accounts that need to be cleaned out. But, the only downtime needed is for (a) hardware upgrades or (b) rearranging the physical locations of the servers in the server room, which involves unplugging the power cord. That means 4-6 months of uptime is typical and then we have to turn them off to avoid damaging the hardware. We had a machine that was pushing 230 days of uninterrupted uptime, and we were all psyched that it might be up a whole year, if we didn't rearrange anything or upgrade it. But then there was a 5 hour power failure that left 90,000 D.C. residents without power. Our UPS's couldn't deal with that and we don't have generator power. Except for stuff like that, the OS could keep on tickin' for years. I have heard anecdotal stories of Linux machines which have been up since before NT 4 was released.
<rant>And yet people persist in using souped up desktop OS's as a server. How anyone could consider something that needs to be rebooted daily to be "enterprise ready" is simple: lack of (valid) information. There is no other explanation. For example, I was forced to use NT 4 for a project, which I might add fell about 6 months behind schedule because of reliability issues. Performance *degraded* over time despite my best efforts to tune the web server and database for maximum caching. Upon seeking the advice of some Windows fanatics I know, they asked me how often I was rebooting it. "Every couple of weeks when it crashes" was my reply. They were stunned and told me, "oh no, that's your problem, you need to reboot it *every night* if you want it to be reliable." So to get a lot of uptime, make sure to crash it before it crashes itself. This is apparently standard procedure in Windows NT installations. Can you say, "cache", boys and girls? Well not in NT land you can't, because every night it loses power and has to be rebuilt during the day.</rant>
It's nice to see Unix making a "comeback", which is how a lot of folks in the media are portraying it. Essentially, it was commonly believed that NT was so easy to use and so closed to being as scalable toward the high end as Unix, that it was a slam dunk for NT to take over the enterprise. Whoops, didn't happen. Linux swept in and took over the low low end (file and print servers, and simple web apps) and Solaris has become the de facto high end unix. NT is sandwiched somewhere in the middle, used by large companies who really want it to work for them in fairly large installations (pushing the limits of what NT can do), and pushed to the desktop. I think it's less of a real world comeback, and more of a mindshare comeback.
Here comes MacOS X, GNOME, and KDE. With GNOME and KDE, IT types can run *nix on the desktop without having to learn to switch the CTRL and Caps Lock keys around and to stomach CDE or OpenWindows (oy). With MacOS X, people who care about usability don't have to choose between a Real OS and one that isn't a pain in the butt to interact with. I doubt MacOS X will make a lot of PC people switch to the Mac, but it may stop the bleeding, and it certainly will bring a lot of Mac people into the Unix fold (now that Steve Jobs has given them permission to like Unix).