Played it after completing doom 2 not having played ultimate doom prior, somebody dared me to do it on ultra violence in the civvie 11 discord. Being relatively inexperienced I worried I was going to hate the experience, then I played it, and a few days ago completed it including the secret levels, and it was an oddly fantastic experience even though I spent several weeks playing it on and off. I found the action be fun, with level design that was often fair with enemy placement to coincide.
Favorite level: Caged
Hard, fast paced with combat that flows well with a massive map that is never overbearing and maze like. Generally the map that for me defines plutonia at it's best, never an absolute pain to slog through with confusing levels laid over frustrating enemy placement, but always engaging.
Most hated level: Odyssey of noises
Plutonia at it's worst, a massive overbearing maze that is so confusing at times that when I think I'm on the right path I'm lead to a dead end where I'm legitimately convinced it's unfinished and have to check and see if there's something I missed, doom 2's city levels are better than this because eventually I finished without having to look up the layout.
Overall plutonia was a fun wad for me, it gets rough sometimes and there's some backtracking every now and again but whenever there is I feel like I'm making progress which about sums the game up. You can be beaten to a pulp and sent back to your last quicksave but ultimately you're always making progress.