Almost finished my first playthrough with only a couple of maps left to discover but I have missed literally dozens of secrets though.
AD is probably the ultimate avatar of classic fps from the pre Half-life 1 era. Seriously.
Its gameplay is refined almost to perfection.
Apart for maybe one or two maps, each level has a very strong visual identity that extends this weird lovecraftian atmosphere that vanilla Quake had.
The level design is truly amazing. It is very typical of old school maps with clever use of backtracking. A few are perhaps a bit maze-like for my low spatial-awareness taste but it is not that bad. Some levels even feature very advanced destructible scenery that you won't find in a AAA game today.
TLDR: you must absolutely try it, even if you are not a big fps fan.
It is thousand times better than any "muh retro vidya" indie 3D shooter shit you will find on steam.