For context, it's a Dark Sun campaign, so heat is a big danger. Having a Cleric who's always walking around with Obscuring Mist provides shade and cooling for the party (at least until I become a Dry Lich, obviously). It also provides concealment to the people around the edges while completely hiding the people in our party that aren't fighter types (and this being a somewhat unusual party, that's common... we've got a non combat librarian Archivist who pops out, uses Dark Knowledge and maybe throws out a buff, then retreats into the mist).
So basically, when we fight our shooters (a Scout 2/Factotum 3, a blasty psion/incarnate type, a shadowcraft mage/cleric) stay at the edges of the mist for partial concealment, our non combat types (my blind Cleric, the Archivist, the SCM when he's not attacking) hide fully in the mist where it's safer, and our melees run out and attack. It's actually pretty darn useful all in all... and originally, it was just a defensive buff for me. We can also have fun with Chain of Eyes... the Archivist or SCM can cast this on one of our front liners, then blast away from the safety of the mist using that for vision.
It's an unusual fighting style, and from a distance just looks like a bunch of melees fighting with some cloud following them, but it's worked so far.
JaronK