I'm not a fan of them, at least not of the ones revealed so far (Pikachu/Zekrom and Eevee/Snorlax). They have colossal HP, built-in Energy acceleration, splashability, versatility and ridiculously high damage output for minimal set-up, as they are Basic Pokémon. It's the Big Basics BW problem all over again.
They're supposed to be balanced by giving up 3 Prizes, and I'm in no way trying to diminish how big that is in a vacuum, but put in context it becomes almost irrelevant as you're taking 4 Prizes with them most of the time. They're not nearly as good against non-GX decks, but then again you can just choose to not put them into play in the first place since you can just splash them in any deck that can accelerate the Energy they need.
We have yet to see what support or counters there will be for these Pokémon, so I refuse to completely dive into the apocalyptic mindset that these will break the game, but right now, with the information we have, I can say I'm not a big fan.