Since I saw Peter Mullen's art a few years ago, he's been one of my favorite fantasy artists, but I always liked his color work better than his black & white stuff.
His new B&W pieces in Dungeon Crawl Classics, though, are inspiring. He's really taken his craft to a new level of excellence. Top notch!
Stefan Poag has some great pieces in DCC too. I particularly like his freewheeling panorama spread across a couple of pages in the wizard spells section.
Some of the other artists' illustrations don't hit my personal sweet spot as hard as Mullen and Poag, but the quality is generally high.
Small text for a small complaint....
The only thing I can point to as a problem (and I only mention it because feedback is the point of a beta release) is that the scans of certain pieces make the inking look unnecessarily sloppy. The issue doesn't stand-out if your monitor is calibrated to a more sane level of gamma than mine.
Not that the above detail looks "sloppy"—I was just trying to show that not all the blacks reproduce quite a blackly as might be desirable, and the lines that you definitely want black and crisp may look a little fuzzy printed at that color depth and resolution.