Well, nobody will really take what I'm about to take seriously, but I'll go ahead and express my opinion. About approximately 5 months ago, Manaphy was tested for OU in the Marriland Forums (told you you were not going to take my seriously), and I had the honor in participating in many battles against the little annoying creature. The experience of being able to use Manaphy in OU, was great for me, devastating for my opponnet.
I used a set that may not be that common, but it certainly did what I wanted it to do the most, attract Blissey and sweep. The set I'm talking about, is the SubTailGlow set, along with Surf and IceBeam/ Energy Ball. With the ability to make 101 subs, and being able to get +2 after just one sub, seems pretty devastating for anybody, not to mention its not an frail subber. A Manaphy with 3 Tail Glows and approximately 50% health, a sub, and EVs for bulkiness, is just killer.
Not many things can counter this thing reliably. I once tried the RDRest set with some EVs for bulkiness as well, and its just devastating when used at the right time, whether there was rain or not. In those times where Manaphy was tested, I used a Calm Blissey with Tbolt, since it reliably counters it, but personally, I find it a bit devastating for the OU metagame, of course nobody has to believe me until its tested.