John Romero it should go without saying is a legend in the FPS world. A Romero solo project (outside id anyway) in the amazing Quake II engine sounds like an instant winner.
This game is ambitious, and it thinks outside the box.
While Carmack was the technical genius, Romero (also a programming wizard) may have been the more creative one in level design. He wanted to make Quake 1 actually far more ambitious than it ultimately landed up being, just a next-gen Doom, but still in the familiar formula.
With Daikatana, he realizes his vision. As it is an experimental game, I find that while it does come from the same FPS tree as the id lineage, it is definitely of a different branch.
Most notorious of this is the save system, which I get why they tried it. Levels are not too long; saves when changing levels; and find save gems which can be stored for manual saves, up to 3 in inventory. This system sounds good on paper, with more strategic choice of when to save, but in practice, you won't know if you should save just now or just around that corner. Maybe you meet an enemy and die. Maybe you wait a long lift ride and wish you didn't save. You won't know till you go around and die first probably (or make it whole way without saving.) I just land up having to re-do the level more often because of this, but it's a pointless gripe since these days mods give you normal infinite saves.
The game interestingly has a light RPG-style leveling system and points for various abilities (str, agro, pwr, etc.) which is cool and unique, without deviating too far from what makes an FPS an FPS, and becoming like Deus Ex or something.
The game's story is also really cool. You're a time-travelling samurai with jedi force ghosts.
Cool soundtrack too.
Ultimately, in story, setting, aesthetic, and gameplay, I find Daikatana to be a really fun and good classic FPS of the late 90s era; a spiritual Quake III if it had been singleplayer rather than multiplayer only.
