Shaymin Formes look good.... but it's all up to RNGesus
The Sky Forme looks awesome. With Hidden Power Grass (or Water to an extent) and Grass Knot, they'll be a force to be reckoned with on the Grass type side. The only problem is, it's a 2/15 chance. (Which is realistic and not impossible like shiny odds) This is actually pretty smart on Niantic's part.... unlike how they handled Girantina. :(
EDIT: There's a 1/8 chance to get it... I forgot that it can get Zen Headbutt too. But when re rolling or if you get HP the first time it's 2/15. It's 2/? because Water Hidden Power is serviceable against Ground/Rock types.
Answers
Shaymin's one of my favorites. It's charge moves look great at the moment but it's fast moves could definitely be better. Since its a mythical/legend, you can expect Niantic to milk the hell out of it. If it's signature move Seed Flare gets released as a fast move, it'll be amazing. Probably the best grass type for Generations.
Likely true, oly possible competition likely would be virizion in gen 5, or maybe tapu bulu in gen 7 if horn leach is added and made good, but I have no idea what their stats/fast move potential could be.
Shaymin likely will be a glass/grass cannon, but that would require sky forme, not so sure how land forme will preform, but if it gets seed flare, and is a good move, I think we can safely say 'Flower power' for the win
Lol I actually completely forgot about Virizon! Looking at her stats on Bulbapedia, both her attack and special attack are a measly 90. Which means that in Go, she will pretty much be Suicune tier regardless of whatever moves she gets (Prolly Leaf Blade and a fighting move as that makes up the majority of her moveset). The only member of Musketeer trio that'll be good is Terrakion. Bulu can be very good, but that's all the way in Gen 7 and it's moveset isn't looking too hot atm.
Regarding Shaymin land, it'll probably perform similar to Mew (they do have the same stats) but maybe a little bit better. Sky form is where it's all at though. Fantastic attack, it's defense is on the weaker side but 225 Stamina is great! It won't be as glassy as say Gengar for comparison who has only 120 stamina. Very excited, but also nervous as to how Niantic will handle it.
Edit: Also Kartana, a Gen 7 Ultra Beast could be a good grass attacker, but it's Deoxys tier frail as hell. So I'll take my cute hedgehog deer thing and be alright!