How valuable is invisibility?

[Ithulta]

For example, how would a ring of invisibility compare with a ring of free action or an indestructible ring of protection (+10)? It is kinda cool to be invisible, but I just got killed by a bunch paralyzing ghouls, so it's starting to seem not too useful. I haven't played much without invisibility, though, so I'm not sure.

[Yottle]

Invisibility doesn't work against high level monsters. It is mostly useful against low level, annoying monsters. There is one quest where you don't want to kill anything- it is great there. Otherwise, forget it.

Free action is much more useful- it isn't even close. +10 protection is also a waste of time.

[Derakon]

Invisibility actually has negative utility for melee characters, since it basically makes monsters move erratically, making them much harder to hit. For blast mages, the extra turns before monsters reach melee range could be helpful, but you should always always have a source of Free Action before you start running into ghouls.

Protection: mostly useless. You'll almost always have something more important to put on a ring slot
Invisibility: you might use Smeagol's ring when you get it, but generally it's not worth spending a slot on it.
Free Action: if you don't have FA from another source, then spending a ring slot on a Ring of FA is definitely worth it.