I may draw fire for this but I've seen a lot of really nice images ruined by heavy handed USM'ing. Edge halos, speckled hair, icy contrast... just too much of a good thing. Frequently the imperfections get emphasized right along with the edges, like skin pores, CCD noise, film grain, etc.
You can sharpen without abusing USM using the high pass filter, it's an excellent way to do it quickly and have a sharpening layer you can tune at will, but High Pass still sharpens the bad stuff so in lots of situations you're still better off with USM and a carefully adjusted threshold.
The deal is, and some of you already know this because I've seen it used here, is an edge mask you can use to leave the smooth areas alone while you work on the edges.
Basically, it's a mask you build up using a copy of the BG layer. You transfer that to a new channel which you can use to select only edge zones. The coolest thing is, you can invert the channel and select only the smooth zones. It's a great way to work on smooth expanses of skin without touching the edges.
Here's an example, it's not one I would actually use; I made it to suit image sizing and JPEG limitations and still look right. (please ignore the huge sensor blob right in the middle of her face... ruined that shoot)...
Chip