As soon as I captured Yveltal, I've used it in about three - four matches for a test run. It was quite fast, as its base Speed stat was approximately ninty-nine. That, if you ask me, is a lot! At level eighty-two, it was able to outrun a handful of Pokémon who were at a hundred. Isn't that impressive? Well, actually, it is a legendary Pokémon, so there may be nothing impressive about that, but still. xD Its offensive stats were quite good, though. Both of its Attack and Special Attack were at one twenty-two, which allowed it to knock out almost any Pokémon with a single hit. Its HP was good, too. Yveltal had complete immunity over ground and psychic, and its only weaknesses were electric, ice, rock, and fairy, which is a total of four. Yveltal's got what it takes to defeat almost any legendary that are as big as this.
I've also used Mewtwo in a few battles, just to see how it feels to have it transform into its mega-form. It was all right, but its Attack power was a little low, though. If its Attack was, say, twenty points higher, the it definitely could've taken better advantage of all the physical moves in its arsenal.