I'm in the process of writing a bulkier post for each of the cards announced but for now I'll just go through a quick opinion of each card.
Gengar-EX: Th card has a lot of potential with some semi-standard, 'Gengarey' attacks (the damage drop etc..). I really like the second attack, as I like hit and run style attacks. Definitely will see its place in the format in some way or another.
Mega Gengar-EX: The attack is really interesting, but the effectiveness of the attack depends on the opponents deck. Sure if they're using a certain strategy against you, it would be wise to use it against them. The difference being their deck is based on that attack beating you, whereas you're just using it because you have the ability to. That's not to say Mega Gengar is bad, its just quite situational I believe. I do like the fact that its not just the active Pokemon you can use the attack of though, that means opponents nay have to be careful about what they bench in certain circumstances. I think its the best Mega-EX we've got so far though, and it will be interesting to see whether you have to apply other effects to the attack you choose to use.
Dialga-EX: Oddly, Im really glad that Dialga is underwhelming. I was beginning to get tired of the Legendary EXs being the best cards in the format, and whilst in some cases they still are, Im happy we're moving towards different Pokemon being the ones to watch rather than just Legendary Pokemon being big hitters. I do like Chrono Wind but Fullmetal Impact (apart from having a pretty amazing name) is just a worse Combustion Blast du to Metal not having any energy acceleration.
Aegislash-EX: Aegislash has an amazing ability and really can stop an attacker in its tracks if dropped unexpectedly. However as nice as the ability is, it can be played around and since most decks don't run more than 4 special energy (there are exceptions of course), a second attacker can just be built up without the Special energy. That being said it could completely lock certain decks and even games into you winning and therefore its still a good card with a good ability. The attack is okay, but not great without the Metal energy intended. However it does make Aegislash splashable and therefore makes it that little bit more playable!
Zubat: Just a standard old basic, its nice that were getting full evolution lines with similar attacks lately though (The Poli-line from Furious Fists)
Golbat: Again, nothing amazing. The fact that Golbat and Crobat both have sniping abilities is good because it means you can drop more damage quickly. Its a shame that we have just lost Devolution Spray however.
Crobat: I really like Crobat, especially as a fun little rouge deck with AZ and Super Scoop Up and things like that. Obviously its shut down by Garbodor but what isn't nowadays. The fact it works perfectly with Dimensional Valley is awesome and I will definitely be trying this deck when its released in the UK and US. The Lightning weakness isn't great, especially if Manetric-EX turns out to be good but at least it has Fighting resistance.
Ditto: I feel with Fighting getting such a boost last set, I really don't think that Ditto stands much of a chance within the format of today. And with the Ability requiring the correct amount of energy to copy an attack, I'm a little disappointed with Ditto. That being said, if Fighting decks begin to decline with the release of this set then Ditto may see some play. That and the artwork is absolutely amazing and I really hope we get some different Pokemon that Ditto has transformed into like we did in the RS days.
Dimensional Valley: Reducing attack costs is always an amazing mechanic in my opinion and in the form of a Stadium, Dimensional Valley doesnt restrict Tool or Bench space which is even better. There are the obvious uses for Dimensional Valley aforementioned but I know we will see some quirkier uses for the Stadium too with Pokemon that no one has really thought of yet. Cards that breed creativity are always welcome to the format and I rally see this as one of those cards, a strong mechanic that makes people think. What more could you ask for?
AZ: The most underwhelming of the cards announced I think, AZ really doesn't do much for me. I don't think its outclassed by Cassius as people are saying, but I still don't think its amazing. With decks that don't run any energy and don't really need Tools and benefit from multiple drops of the same Pokemon, yeah it's great. However I can only think of one example of this and that's the new Crobat Line. So yes, AZ will see play in Crobat decks, but as of now I don't see much else benefitting too much. Though Full Art AZ will be awesome.
Mystery Energy: This is the card I am happiest about. Not because its amazingly over-powered (because I don't believe it is), not because it's a different effect from what we have seen for a while. It's because we are getting specifically typed Special energy cards, a concept which I think is far underused. I just hope now they have done three new types of Special Energy, they continue the theme with the types that are missing as of yet. Mystery Energy itself is good. Nothing more. It works perfectly with Gothitelle and a couple of other cards, but other than that I don't see it as being as good as Strong Energy.
As I said at the start of the post I am writing up a bit more of a bulky description of the cards described with some specific strategies. I don't know when or where it will be posted, maybe when the full set is released. Who knows!
~ Jack, OmniPoke