Green is definitely one of the stronger colors in EDH, being the color of ramp and fatties.
Seriously, though, he seems pretty playable. I'm not a fan of the mana cost, but the ability spread is versatile enough to keep him from being a dead draw in a lot of cases. Not that there's any real value in saying it, because it's true of damn near every planeswalker, but, DANG that Ultimate is awesome. (Worth noting: Doubling Season does allow him to Ultimate immediately.)
Green actually provides some compelling new options. Overall, it's probably the strongest color in the set, at least new-card-wise. Having three playable Mythics is quite a home run.