I just finished what there is of Black Mesa (it goes up to entering the portal to Xen) and there's really no two ways about this. It's fucking awesome. It's still Half-Life, jumping puzzles, man-shooting and a bit of jank. They've managed to remove quite a bit of the jank, most particularly by remixing "On a Rail" entirely to be a good level, but they managed to add enough jank here and there that it evens out.
As far as a graphical overhaul goes, the term doesn't do this thing justice. They rebuilt the essence of almost every single area in the original game from scratch, made it outstanding to look at, and somehow keep the same feel in both gameplay and tone. Even the remixed On a Rail feels like the same sort of level in tone, it just doesn't suck 30 miles of electrified track anymore.
I can't wait to see what they do with Xen.