Some of the best Quake out there. The quality of execution in the design, especially the level design, is so good it's kind of beyond me to meaningfully critique. Hell, I beat Foggy Bogbottom without getting lost, that's a feat of level design right there.

The main thing to remember is that each map is its own, fully supported, fully complete experience. I haven't gone through everything yet, but the half I have played are massive, intricate, and complex. Hit these things one-at-a-time when you need that Quake fix, and you'll have a good time.