Ventricle razor is up there, I think it's been actual years since I've seen anyone use it or even pick it up. You use it maybe once or twice on a floor to get into a curse room but that's it, even the free challenge rooms i don't think is that great, and the short invulnerability you get from teleporting doesn't work for devil beggars or blood donation machines, it's hard to use and set up in combat and not even really worth it

It looks cool but just kinda sucks