You get more volume and loads more clarity - and a few gadgety sound "modes" like Spatial and Electronic.
Downside - harder to upgrade if you want to change head unit, amp or speakers.
The advise is always - get your favourite music and listen to an HK system at your dealership. Compare with non-HK. If you think HK is perfect, get it. If you think you might want even more, start with the basic system and go aftermarket.
It all depends on what you want. I have it, or should I say had it, and am currently going threw and replacing it. I guess it's pretty good for the money but there is deffinately better sounding stuff out there. Oh and if you do replace the head unit on an HK mini and keep the factory amp then be prepared for the floor noise. Lost me 2 points in my last SQL contest.