If it weren't for places outside of the game (like this thread), I would have never known they exist (you never hear about it in the game).
I just don't understand the point of them. If people find it fun, that's fine. But one of the biggest purposes for an MMO, is to get a tangible reward, and progress your character. Challenge modes provide neither of that.
I think Blizzard is on to something with non-LFG Heroic scenarios. LFG should keep the same crappy blue, easy mode dungeons. And the "heroics", should be pre-made groups that were Cata launch or BC difficulty that provides epic loot. With this they can get rid of ridiculous challenge modes and scenarios.