I'm still somewhat neutral on metagross as I've read quite a few good arguments supporting and opposing its ban, and I'm still trying to establish my own opinion on its effect on the metagame. Lately this has been difficult since metagross isn't allowed in OU, so all of our theorizing and arguments are based on past experiences at this point. However, seeing as Mega Metagross is strictly offensive it's a bit easier to come to some sort of a conclusion, because we don't have to account for all kinds of different sets. As far as I'm aware, only its coverage moves change with the occasional Agility or Hone Claws(?) set.
So since Metagross is an offensive wall-breaker, I assume that its worst enemies are things that can switch into a predicted attack and survive a subsequent coverage move, while threatening Metagross with a strong attack or crippling status move. For purposes of gathering some concrete data, here are the relevant pokemon in OU (S, A and B ranks i guess) that are capable of switching into a STAB and posing a threat against the dual STAB + Hammer Arm + Grass Knot set, which I believe is one of the best sets.
Switch into Meteor Mash and win 1v1:
(life orb),
(def or scarf),
,
(twave),
(with def investment),
(def or scarf),
(def),
(scarf or sand),
,
,
(def),
(reflect type),
(def),
,
(def),
,
,
,
(scarf),
(band)
Switch into Zen Headbutt and win 1v1:
,
(max def),
(life orb),
(def or scarf),
,
(twave),
(with def investment),
(def or scarf),
(def),
(scarf or sand),
,
(def),
(reflect type),
(def),
,
(def),
,
(def),
,
,
(scarf),
,
(band)
Whether or not these are enough switch-ins to neutralize Mega Metagross's influence is subjective. Pretty much everything that isn't listed here is either really unreliable (ie def zapdos) or cannot prevent the 2HKO against this particular set. Something else worth noting is that a lot of the pokemon listed above cannot afford to switch in more than once or take any prior damage (ie def heatran and ferrothorn), meaning they are also shaky checks. So this isn't really a list of checks and counters - it's more a list of everything that can possibly stand up to Metagross by switching in. It's not a huge list.
On the other hand I know I've stated that having 9 resistances and great bulk allows Mega Metagross to come in a lot, but I've noticed that Metagross still has a difficult time finding an opportunity to switch in directly. OU is well equipped with moves to keep steel types in check, and Metagross's secondary typing does not mitigate any of Steel's weaknesses. However this doesn't stop Metagross from coming in every time something faints, in which case it's still a revenge killing monster along the same lines as Greninja. Only it's a bit more difficult to handle.
I don't know if Metagross is too powerful or too good or whatever, but I believe its influence on OU is ridiculous and it puts a lot of strain on teambuilding. Its power, speed and coverage allow it to plow through almost any team that doesn't have at least a couple of the switch-ins listed above. It's one of the easiest and most effortless win conditions assuming you don't miss.