The first thing I noticed was that A: it gets better as you level up, and B: it is very fast with rest state. At first, especially in Outland or Northrend, the dungeon queue is populated with utter fuckbends: prot warriors queueing as DPS to level their weapon skills, ele shamans pulling without end or pause, and healers who afk without a warning. After a bit though, once people have a decent set of the new gear and have done the initial dungeon quests, things get much better. Many more people are familiar with doing dungeons and runs become smoother and more consistent (though locks can never seem to NOT Drain Life).
As for experience, here's an example. At 72, I get about 10-12% of a level from doing AN, which takes 15 minutes (no heirlooms). A full level at that pace takes about 3 hours, assuming a shit group or two. That is crazy hot. The only problem is, all that XP comes from mob killing, which takes your rest state down fast compared to questing. Without rest, it takes about 6 hours, the same as quests, but substantially less profitable.
There's another casualty here, besides rest. It's the zones themselves. There are tons of awesome quest lines, like Wrathgate, Sons of Hodir, D.E.H.T.A., Drakuru, and more that get tossed aside for the speed of dungeon leveling. It's more than awesome when you're still stuck in the shit Old World, but in Northrend, you're missing out on a lot if you get dungeon tunnel vision.
So here's the upshot, from where I'm sitting: pure dungeon leveling is advisable only under three conditions. One, you are a tank or healer. Two, you only play a few hours a week (for rest to come back). And three, you've already ding'd 80, having seen a lot of the content. Even with that said, there's a few quest lines that you simply shouldn't skip, like the one that unlocks the Shadowvault. And dungeon quests, natch. And anything that jumps your rep up dramatically, like the initial Mag'har line. And of course, anything to do with poo.