The short version: I hate them, as I hate Hell, and all Montagues.
The not-as-short version:
I recently came down on campaigns where there is a set ending toward which the players are gently (or not so gently) guided. On a related subject are games where the setting"develops" through updates found in the splatbooks. No, White Wolf isn't the only company guilty of this; Dream Pod 9 did it with Tribe 8, TSR did it with Dark Sun, and so on.
I despise metaplot for a number of reasons. Firstly, metaplot tends to become "the story of a bunch of badass NPCs that your players might, if they are lucky, get to watch.. They are Luke Skywalker, and you are Red-4 or Rebel Commando #3 or whatever. While I no longer gravitate towards "hero with a capital H" style gaming, neither am I satisfied to play a game where the only important things that are happening are instigated or shaped by NPCs.
Secondly, metaplot tends to waste valuable page count in books. Yes, I can ignore the meteplot, but this renders many parts of many books useless. Maybe I'm interested in the revised mechanics of the new setting book, but I want to use the old setting before the Big Evil Guy was killed off and a bunch of shit was arbitrarily changed... and I can certainly do that, but I'm also paying for that material that I don't want. (This, in recent times and certainly going forward, will preclude me from buying "revised" settings.)
Thirdly, and perhaps this is just me being persnickity, but almost every metaplot I've ever seen has dropped the ball in some way at or near the end. The ending is illogical, poorly thought out, etc. Metaplots also tend to advance the setting in ways I find uninteresting. I freely admit that point three might just be my general bias towards metaplots.
I do have one notable exception to my general metaplot hate:
In Nomine, the Revelations campaign. The actions that characters take can actually influence the setting in ways that are not contradicted. For instance, the players have the chance to save an Archangel from spiraling into a fall from grace, and the book presents write-ups on the Archangel redeemed and the Demon Prince he becomes if he should fall. The players have the chance to confirm/prevent the ascension of another, more minor, Demon Prince as well. Finally, the players can prevent Armageddon, or they can usher it in. (Though on the flip side, it is *very* difficult to cause Armageddon as opposed to stopping it... difficult, though not impossible) No further campaigns were ever released for In Nomine, so changes to the setting were never contradicted. (Really, if you think about it, what campaign could have possibly topped the aversion of Armageddon?)
Even when I was into games with more story/narrative focused bent, I never like metaplots... back then, the reason was mainly due to the fact that I wanted to tell my own stories. Now, I want a game that drops a setting in my lap and lets me run and play. Give me more options, I might buy them... give me a great module, I might drop it in, but don't fuck around with the setting in ways that make me have to ignore/reconcile the published world vs. the one I've carved from it. (See also: my gripe about Deadlands and its totally fucking UNVINCIBLE* pet NPCs that they straight up ask you not to kill so as not to muck around with their publishing plans.)
*Yes, unvincible, which is way less vincible than invincible. Don't believe me? Look up the stats for Stone and Raven and Reverend Grimm. Seriously. Why bother with the long, atrocious stat blocks when they could just save space and print You lose! Good DAY, sir! after the descriptions.
Oh, and Elminster can suck it. So can Drizz'zzt'ttt or whatever the hell his name is.