It's super neat. Map quality is all over the place, but most are real gems. I've only had one soft-lock in about 20 maps, and only a handful of those had impossible to beat final fights (I'm sorry, but failing to take down 15 shamblers at once, in a room with four central columns for cover is not a "skill issue").

In fact I never heard much about Quake having singleplayer.

It had good singleplayer for the time. IMO, it hasn't aged particularly well. ID was learning how to do a fully 3D game on the fly here, and it shows in spots. The best moments are built on experience with building Doom maps, but that's practically a different sport.