I'd give it an 8/10
While I'd say it's best towards the middle half of the game, the fact that it does 20 when you first get two energies on it, then 50 the next turn unless they heal, makes it a great first or second pokemon. I'm a bit confused by its poke-body though, how can it be an evolved pokemon and still keep that power? But it's cool that if you get attacked before you get your second energy on, you get to do 50 in retaliation.
I can't quite make out the rarity of it though, anyone know?