I think Mac OS X is much more user-friendly than XP, and is, out of the box, a closed system nearly impervious to hacking. From a serving standpoint, I have no experience with OS X Server, but I would assume it is just as secure as OS X itself.
Linux and *BSD are very secure in the right hands, and are inherently more secure than any Windows variant. While a Windows box can be stable, the amount of work required to achieve a result similar to a Linux/OS X/*BSD counterpart makes one wonder if it's worth it.
I wouldn't go as far as saying that OS X is nearly impervious to hacking.
MS OS's are used by the bulk of PC owners, and is therefore the most appealing in the eyes of those that like to, ummm, find exploits.
If OS X was on the bulk of the PC's, I'm quite sure you'd be hearing about this exploit and that exploit, and MS would would hardly be mentioned.
On a side note, I read where MS will no longer be updating Outlook Express. It will set there and collect dust now. Their focus is on MSN and Hotmail along with Outlook. Anotherwords, they are saying: "Show me the Money" yet again. Oh, they have also stopped supporting Windows 98 and 98 SE. There will be no more patches or service packs. If you have some that you haven't yet installed, they will still be listed, but no new ones will be coming out.
I think this is called... Hmmm... A forced upgrade$?