I think it's the expansion that made me really love Hollow Knight since I love to optimize boss fights, finding a way to be better, see the flaws of what I'm doing.

Pantheon of Hallownest was tedious at times since it was a bit frustrating to start from the get-go after being defeated by a boss like Oblobble when I was not careful enough and just wanted to get to the end (shit boss by the way) but you really feel your progression as a player, you become more constant, and you feel the pay-off when you beat Pantheon of Hallownest.

I spend 60 hours on this dlc (a lot more than it took me to beat the normal ending) and it was a blast, it reminded me the feelings I had when I tried to beat Bloodborne with a LvL 4 character, the feeling of overcoming a really difficult challenge.

The expansion also add Pure Vessel, probably my favorite boss of the entire game, or at least my co-favorite, I can't decide between him and Nightmare King Grimm since I think Pure Vessel is more interesting since the rhythm is more difficult to master and I also really enjoyed the very melancholic mood of the fight, but Nightmare King Grimm is more spectacular and epic while being a fight giving you less creativity than Pure Vessel in the way you are playing (since it's basically a dance with him).

Absolute Radiance is also an excellent fight even if too reliant on RNG (the boss can be hell or very easy depending of the RNG), it's a very fun boss to try to master since there are so many combinaisons of patterns, you always have to adapt to the boss. I fought her countless times and never a fight was similar to another one. The Nail Masters were also a very nice addition, Sly being my favorite with the Brothers.

It's just a shame that some bosses are poorly balanced, I already spoke about Oblobbles but Markoth without floor is boring, No Eyes is not really a hard boss on P5 but the boss can be really annoying to get stuck in places you just can't hit. Sometimes I'm wondering if the dream bosses in P5 were tested before the extension was released.

I can't rate it alone (like Grimm Troupe DLC) since it's part of Hollow Knight for me, but I really loved it individually and for how it impacted my Hollow Knight experience despite the flaws I wrote about.
It's only a DLC for really dedicated players who love to tryhard, I don't think it's a fun DLC if you don't like this kind of challenge.

The fifth pantheon is a nightmare with 90% of the bosses beeing "filler" i consistantly did up to NKG no fail, but always failed in abs rad or pure vessel with each run up to 40+min this pantheon is an journey apart. you literally have to study the bosses and master them, dying in anything past markoth or gpz hurt your soul but by the end even pure vessel will be pathetic, but dying to abs rad is even worse my god... i love this game but i will not come back to this pantheon NEVER AGAIN.
10/10 dlc. 10/10 game.