Actually, the Choice Band Zapdos in the Analysis has Giga Impact (to take on Blissey of course).
IMO, they're decent. I kind of wish they worked like in R/B/Y days, but wishing does nothing. I can see of SOME use for it, but not much to warrant use.
There is only a total of 4 of them. 2 of them are exclusive: Dialga with Roar of Time (useless, but cool-looking move) and Rhyperior with Rock Wrecker (might be an ok thing under a Sub... not pushing it though).
I guess it seriously depends on the OHKO / 2HKO range. Just like discussed with Porygon-Z, it DOES have reasons to use Hyper Beam. Slaking, on the other hand, could make some use of Giga Impact, but you have Double-Edge anyways, which in the end does still some superior damage and allows you to switch out.
It basically comes down to what's better: a move that "technically" does roughly 75 damage per turn (more for STAB, however), or just to stick with something that does maybe 100 damage / 120 damage / etc. in a single turn? Is the "Hyper Beam move" going to KO something, while my other move is only 2HKO'ing? Will I be set up on the next turn? Will a counter occur the next turn?