Legendary Pokémon

Alright, we have a major addition to the metagame in the form Shaymin-S. Its SAtk is Base 120, it has a base 127 Spd, and a base 103 Atk stat. This is obviously a versatile pokemon, capable of Attacking from both sides of the Spectrum. Its stats allow it to outrun its best revenge-killer: Weavile. However, its defensive stats are not Uber-like at all.
Its move-pool gives it powerful and useful STABs, but its move-pool is very, very shallow. Seed Flare is its strongest and most dangerous move, dropping the opponents SDef stat 2 stages 80% of the time. It also has Serene Grace and Air Slash, making it difficult to counter if you cannot outrun it. It does have Swords Dance and Growth to further its damage output.
It is a Grass-Flying type, which makes it weak to 3 common types in the OU: Rock, Ice and Fire. Its Rock weakness makes it weak to Stealth Rock, which is a common entry hazard in the OU battling scene. Its x4 Ice weakness is the best place to attack. On the off chance it does not carry a Yache Berry, almost any Ice move will OHKO it. And as I said before, its defensive stats are nothing like an Uber (they are worse than Garchomp's!)
As with all pokemon, it is counterable. Blissey and Regice are the two best candidates. Blissey can Thunder Wave / Ice Beam Skymin and Regice, well, it is self-expanatory. These two work well even after there SDef falls (particularly Blissey, who will probably not be 2KOed even after the drop.) And besides, if you have two pokemon that stand up well to Special Hits, you can switch between you wall and the other pokemon, nullfying the drop and making Skymin waste its 8 PP of Seed Flare.
There are a whole slew of Revenge killers that can rid of Shaymin-S, the big ones being CS / Ice Shard Weavile, Steadfast Lucario, Steadfast Gallade, Agility-Gross, CS Azelf, CS Infernape... The list keeps going. I personally believe that Shaymin-S is not as scary as some people claim it is.

So the question is... Shaymin-S: Uber or not?

In my opinion, it's an uber only because you can't kill it without having a pokemon that counters it. I mean, when I make teams i never choose pokemon in order to counter others, so when i fighted(?) with a shaymin sky form I got sweeped quite easily (My walls would have killed it, but there was this flinch thing).

Not versatile enough if it reverts to original forme when on WiFi or battling with D/P though :/

That, and the fact that it cannot be Skymin at night or if frozen...

I'm not voting right now, I am not sure at all whether it should go to the uber/OU tier. I've made a team in Shoddy for testing some of Platinum's additions and Shaymin-S seems to kick some butt, always saving me from difficult situations. I still haven't met a Pokémon that can counter 100% Shaymin-S (in OU that is), mainly because it can flinch-hax everything out. I've faced quite a few of them, and they did give me trouble, however, it's not a Pokémon that can really walk through anything and not get killed. Now, I'm closer to voting for OU, but I still dunno.

By the way, random thought, but I think Yanmega is quite a nice counter. Switch in on anything but Air Slash, use Protect, become faster, hit with HP Ice. But yeah, that's just theory.

I vote that its an OU pkmn,because it has 4x weakness against ice type pokemon

About Yanmega...

It is a nice counter. It can take 2 LO boosted Seed Flares from a Max SAtk, Modest Shaymin-S. It is also immune to Earth Power, which it is almost guaranteed to carry. And if for some reason it is running a Choice set, it can switch in easily if it chooses Earth Power or Seed Flare.

The key to countering Shaymin is to be able to outrun it. Unfortunately, it is the tenth fastest pokemon in the game... And now that Deoxys-S is in the Ubers, that does not help. But you still have the 394 Speed Freaks, and it looks like Jolteon and Crobat, the latter in particular, can counter Skymin with their decent SDef and Super-effective moves (HP Ice and Sludge Bomb.)

definatly . its so op

Well,I think Shaymin-S should be an Uber.It has 2 great STAB moves with nice side effects,and it has a good base stat for Sp.Attack and an even better Speed.It can destroy almost everything woth some lucky flinces from Air Slash,as it can with Seed Flare's Sp.Defence lowering.

I personally vote for OU. I know it's a pain to counter a Shaymin-S if it outspeeds you, but on the other hand, if you' re faster than it, you can OHKO it. And that's why I doupt whether it should be a uber with all the freaking special walls that exist thre. eIt may be a good sweeper, but I'm sure it wouldn't stand many chances against Deoxys A, Mewtwo, Lugia, Ho-oh etc. That's just my opinion though.
About the flinch hax, I know it's annoying, but it won't always work. Any pokemon capable of taking a couple of air slashes would be able to KO it afterwards.

I vote for OU.. Shaymin-S has great stats but it's not unbeatable. If you're faster that it like Grovyle_Fan said, you can kill it. There are many Pokemon that can kill it, like Aerodactyl or Crobat..

Well,it is like it forces us to use some specific Pokemon just to kill it.

Uber i think, due to the listed EA reasons

Shaymin-S is alot like togekiss and togekiss is not uber so niether should shaymin-S

Btw that's not a reason. And Shaymin-s is way faster than togekiss. And i'd vote for OU also because of it's weaknesses. 2x fire, 2x Flying, 2x Rock ( i think maybe sthing else ) and 4x ice. These types of moves don't lack in any team especially fire, ice and rock so if a pkm xan resist its attacks its all over