I have very few actual thoughts on dual spec, other than that it's a Good Thing. I can't say I'm super-excited about another cash sink on many of my alts, but I'll manage. For me personally it'll save me a lot of respecs and reglyphs going from tank to DPS and back again, though I can't see as much utility for it on my warlock (maybe solo spec vs. raid spec, I suppose). I'm going to point you at Siha instead since she said pretty much everything I wanted to say.
I will add that I think dual spec'ing is like Blizzard's apology to hybrids. The manual (yes, referencing the vanilla manual again, since I still have it - dwarf mages, plainsrunning and all) implies that hybrids should be able to dps and heal and (in some cases) tank; however, it's been a long accepted fact that trying to spec for two roles using the same pool of talent points results in subpar performance. While you can't switch midfight, at least now you can switch between fights.
There you go, hybrids: it's only been, what, 3 years?