For me the Mini was a way to dip my toe in the Mac waters, to learn about OSX, and to see if I liked it. I ended up deciding very quickly to make the Mini my primary machine even though I'd been working on a much more powerful PC I built myself.
The other advantage I see to the Mini is that I tend to upgrade computers much more frequently than I upgrade monitors. With a Mini I can continue to do this.
I'd like to upgrade to a more powerful Mac, but now that the 2012 Mini has discontinued the discreet GPU, I'm not sure what my upgrade path will be. The Mac Pro (unless it's discontinued) is the obvious choice, but it's very expensive, about twice what I paid to build my own comparably-powerful PC.
Anyway, for the OP, it really comes down to your own personal needs, and what type of platform you want (e.g. an all-in-one or a system with discreet components).