[thoughts summarized]
Expert Belt will be a blessing to things that are already meant to be impossibly difficult to knock out (Scizor, Torterra) because the two-prize penalty is less worrisome. I would be a little more fearful putting it on Kingdra than I would on Scizor, but not much. I think it's a waste on any Gengar (should be obvious), Machamp (also kind of obvious), and Beedrill. I think it's not worth it in Plox or Flygon decks for the cost. I may be wrong though (80 damage plox is kind of upsetting]. I think that the same goes for Gyarados, but I'm a lot more hesitant to say so, since it's absurdly fast and powerful even without Expert Belt. Salamence will love to see it in the metagame, though I would not be surprised if Salamence itself also ran it from time to time (I might run two of them in my build).
The metagame's stage 2s were already so ridiculously fast that they could compete with SP decks. Now that Expert Belt gives them an extra HP and power boost, lots of things will be pretty much impervious to anything short of perfect SP builds. You'd need an epic BlazeRay to beat down a EB Kingdra deck. Is it possible? Yeah. Is it likely? Nope. Same goes for Gyarados, and the only reason Scizor isn't included is weakness (which even then isn't quite so helpful; Scizor's 120 HP survives Jet Shoot unless it's burned).
I think Expert Belt was designed to help ease the passing of SP decks, when the format rotates and we find ourselves suddenly without them.