I picked up both X and Y, and while I absolutely love them, they're also a bit disappointing.
The chiefest failure is the story. Game Freak has attempted to make the plot less of an excuse, but after the excellent plot of Black and White, X and Y are plagued by a predictable villain with motives that aren't reflected by the game's world, poor character development, and many wasted opportunities. However, the ending is satisfying, and the character of AZ has a poignant story to tell.
The difficulty is also problematic. Pokemon games aren't known for being tough, but the game flat out hands you an item that will cause you to be ridiculously overleveled even with a full team of six. I didn't grind, but after a while I was at least ten levels higher than all my opponents.
Also, while I love the concept of Mega Evolution, I find the designs tend to focus too much on being generically "awesome", with random spikes and such thrown in with no thought to what made the original designs so appealing. And while I was still eager to play around with Mega Pokemon, over half of them are only available in the post-game, and the method of finding their stones is needlessly obtuse and has no reason to exist.
That's not to say I don't adore the game. While lack of readily available numbers hampers Super Training, it makes EVs more accessible than ever before, and Pokemon amie makes me really attached to my mons. As well, improvements to the GTS make it actually useful for completing the Pokedex, and Wonder Trade has to be one of the greatest ideas to ever grace the series.
Also, I feel that the new mons are some of the best aesthetically. Despite the general dearth of them, I love nearly every one of them and am eager to try them out on my teams. And many of the old mons look better than ever in 3D graphics. The mons make for gorgeous looking battles.
At the end of the day, it's not everything I'd hoped it would be, but it's still pretty enjoyable, and contains tons of improvements to the formula. Oh hey, there's always Hoenn (totally confirmed by the way) to improve even further, right?