There are some really strong cards in Great Encounters, to sad that most of them are weak to the dominating Meta decks.
Let's see...
Porygon-Z
--> I already tested that decks, and with the right trainer engine, it runs really great - except for the big problem that the attack depends on luck, but you can still assert (87,5%) that it will do at least 80 dmg with Power and Lake Boundary. But a pure Porygon-Z deck has a VERY bad matchup to Gardy/Gallade, even with Crystal Beach and luck its hard to win, not only because of Porygon-Z's fighting weakness, but also because Gardy blocks the power you need to attack while it can use all of your rapidly discarded supporters.
Sceptile
--> There are several nice decks you can build up with this card, all of them lose to Magmortar most of the times. Typical grass Pokémon have fire weakness, and there is Dragonite ex which does 40 damage per Deafen lock, Magmortar heals 20 per turn and is able to snipe Sceptile, making it difficult to rely on Dragon Roar.
Blaziken
--> yeah I think this card could make some States. Although it's still overshadowed by Magmortar, it has some nice adventages.
Darkrai Lv.X
--> Really strong card, but goes down too fast. Most players will tech against status conditions because of this Pokémon, and there are a lot of ways to do (Unown G, Holon NRGs, Switch/Warp Point etc.).
Togekiss
--> This is absolutely overrated. I didn't play often enought to give proper statistics, but I played a deck with 17 basic energy and still got only 2-3 on average. The power is great, but once it's on the bench, it can only be used to buy time because of its crap attack.
So personally I think PUI didn't very well at their choice for GE cards, and I'm afraid the current meta decks will still go on domination the tournaments.
--> Porygon-Z is a strong card and will be played a lot, this will help G&G (which completely owns it) to keep its position.
--> Sceptile also is a strong card and will be played even more, this will help Magmortar (which completely owns any variations) to keep its position.
--> Darkrai Lv.X will make Blissey even more dangerous to hold its position in the GE metagame.
It would be that easy for the PUI to break these terrible metagame chains, but it seems that they have failed. Glaceon Lv.X would do extremely well against both G&G and Magmortar, Kabutops w/ Fossils is very strong and has no death matchups to those metagame decks (if they don't run LatiLock).