I'm the first to admit that the mac mini sort of feels like a nichey product, and if it were made by the likes of Dell, HP, Samsung, Lenovo or any of the other major OEMs, then it would probably be a dead-end product. But it's not. It's an Apple-branded PC, and at $599 it's the lowest step onto the Apple ownership ladder.
Sure, the latest upgrade is the bare minimum that Apple could have gotten away with (and remember that the old Mac mini had been around for years ...), but it still appeals to people. For example:
People with a sub-$1,000 budget.
Those who want to take the Mac OS for a test drive.
People who are space-limited or have plenty of spare monitors and keyboards.
While I'm ready to admit that the Mac mini is an odd nichcy bit of kit that looks a lot like a biscuit tin, I've had one in the lab for about 16 months now and the ownership experience has been a flawless one. The Mac mini is a nice, quiet system that just works.
No doubt there are more sensible ways of spending $599, but a Mac mini is still a pretty good buy.