It's a bit of both. I took on one of the longest challenges and completed it, yes, but also, it does have value beyond that. I plan on getting the full Brimstone set, but I don't want to do 6 bad moon runs in a row, and playing HC cloverless is annoying. Also, if Jick ever adds BM-specific trophies even after I get the gear, I can now switch whenever.
As Galeesa said, it can be frustrating and time-consuming, but in most cases, it's not actually all that difficult, as long as you do it as a Seal Clubber, which depends on items and skills so rarely that the cat can't interfere much. The only big issue I had was with the sorceress (the cat and sorc both blocking skills SUCKS), but by going out of my way to do things like the blackberry exchange, I *was* able to kill her at level 13... and yes, I forgot to discard my instant karma. Again.
Unfortunately, black cat is the only challenge in the entire game that does NOT end when you break the prism, so I had to insta-ascend. I'm starting my next run with a clubber and a volleyball to get back up there quickly, however, and I'm going to be dual wielding a brimstone club with a nemesis weapon, so this is probably going to be the shortest run I've done in a looooooooooooooooooooong time. I definitely want to join a clan dungeon run, just not with the cat. Hobo bosses with the cat blocking stuff fall firmly in the not a chance category.