So, I was messing around to see if I could get any use out of this item's various effects and I discovered that the Naga Cauldron has a sneaky little bonus thing attached to it. It allows overheal. Priest potions heal 100%, so you always get some overheal out of it.
This means that a Priest with a Naga Cauldron can heal a base of 100% up to maximum overheal health (like, 150%, I think) with every glug. Even with no status effects. I've no clue whether this makes up for the opportunity cost of having another item that is actually good but it does make the Cauldron at least an interesting option for a Naga City run or the like
Edit: OK, so, a Halfling Priest of pure JJ with the Cauldron can beat Naga City (not least because on that map you can use Chaos Avatar then pick up exactly one point of weakness and corrosion before heading down), and my best use for it is a sort of alternative healthmonster build with 2 shots of bonus health, 1 of bonus mana and some gleeful glugging.
The primary advantage of this build over an alternative JJ priest build is that you don't waste any of your health potions' effectiveness (and, being a halfling, you will have *tons* of potions), and the little boost to mana potions is also quite nice. My attempts at human/taurog with the cauldron weren't as successful. I think it probably beats bringing your own Keg of Health along in terms of efficiency and the number of extra hits it lets you take.