I'm not surprised by Shaymin EX's popularity at all. I've been playing it even in this format since Regionals, and all I've come to see is that it's an amazing, underrated card. You just have to stay careful about when you have it on your bench and how easily your opponent would be able to kill it, as with any other EX. However Shaymin is the frailest of all EXs so you must be extra careful. That being said, it's an amazing sweeper late game outclassed by none if you use it right, and you can tech in one grass energy (if you run prisms) so that if you're stuck starting with it you can net an extra energy out of it.