Gotta say, the artwork on both cards look far better than the scans we initially saw in the Japanese leaks.
Mega Diancie is cool with a cool attack effect, but capping at 190HP on a Mega with no access to Spirit Links makes it a vulnerable target to Heatran, which can OHKO it. You can still combo it with Aromatisse and Substitute Robot, so it balances out to be good.
Gallade looks at least decent with the help of Dimension Valley. 3 for 130 is okay, and Assault Sword can target down a utility Pokemon and take an easy KnockOut with a Lysandre. The following is a list of all popular cards that can lose easily to it:
It takes out Expanded Eelektrik, Aromatisse, Spiritomb, Mewtwo-EX, Night Marchers, Raichu, Dedenne, Meloetta-EX, pretty much every NFE card, Garbodor, Electrode, Mr Mime, Deoxys-EX, Toolbox Sigilyph, Victini, Dragalge, Miltank, Machamp, Klefki, Golbat, Wobbuffet, and Both Slurpuffs with Muscle Bands.
I actually plan on using Christmas money to maybe get this collection, because I really like the card, and I still have a lot of cards to find to complete my XY, Flashfire, and Phantom Forces collection. All the more helpful by the fact that this is only a $19.99 product, in comparison to the $49.99 demanded for the Mega Metagross Collection.
Edit: Misread the card that Assault Sword does 60 more, instead of 40. It's a bit less playable then I initially thought, but it still looks fun.
It is peculiarly funny that Gallade-EX and M Diancie-EX released at the same time, and they fight each other in that ORAS Anime Trailer.