So I just read, maybe I had seen it before but it just didn't register, that Challenge Mode dungeons in MoP will use LFG - personally, I don't think I'll ever use LFG for that, can you imagine the crap that tanks and healers will take in those. I can just hear it now.... You'd need a whole the size of the Grand Canyon in your head to tank or heal that viper's den.
Yeah, that's how I figured I'd use it. If going solo, I'd go for the usual heroics, and in MoP they'll be WotLK-style; i.e. puggable in LFD. If I want some fun with guildies, then we make a premade group and hit the challenge mode.