Well, in regards to reliability, Apple's are very high. Thats mainly because they control tightly hardware and software certification. Thats not to say you cant sabotage the stability of the system by installing grassroots apps and all, but overall, its nice and stable.
Word of warning tho, when OSX goes down, it goes down hard. Intricate knowledge of BSD or Macs arent usually enough to save you. hehe. I have had to help pull many a dying Mac from the brink from very Mac and OSX saavy people. A good understanding of BSD will help you out a lot in customizing the system more so then just what the GUI will allow you and help in many bad situations you may land in.
Upgradability, the Mini's from what I have seen there isnt a whole lot you can upgrade simply because theres no room. The G4 and G5 towers tho, you can upgrade damn near everything and install a slew of add-ons. The self contained systems (Imac, Emac, Mac Mini) theres not a lot outside of HDD, Memory and sometimes *-ROM drives that you can upgrade only because they are designed that way.
Its a "different" type of machine, not nessisarily better or worse although it may exceed windows or linux in some areas, it is beaten in others. Id say it makes for a solid OS foundation with a simplistic but flashy and customizable GUI. Unlike Windows which seems to have a more complicated GUI with every new version.
Also, in regards to security, Apple is being heavily critisized for downplaying a lot of its vulnerabilities to its users and being slow to patch. They are often not forthcoming about how bad the holes really are when you go to update. Good example is when they had a big hole in Quicktime that allowed code execution and called the update a "performance enhancement" to thier users. Just an FYI.