Having finished X / Y, I definitely have an opinion on it. I'll just say this: I haven't liked and been so engaged in a Pokémon game since R / S / E! X and Y are quite strong titles and they are very enjoyable.
What surprised you as you played through the game?
Wonder Trading. Damn I love this feature! Wonder Trading makes trading Pokémon so much easier. Even though the mons you receive are always random, I have gotten some really good stuff through Wonder Trade, including a 5 IV'd Rotom. I definitely hope they keep this around. It's very convenient and the fact that you never know what you get makes trading with people more enjoyable.
What disappointed you as you played through the game?
Probably the biggest disappointment to me was the Pokémon League. This is supposed to be the most intense part of the game, battling the Elite Four one by one and receiving the Champion title. I found this Pokémon League to be a joke (and I didn't even use the Exp. Share in my play through). The Elite Four weren't too hard to beat and Champion Diantha is in the same boat. Her Mega Gardevoir was the only mon in the League that presented a challenge to me, but otherwise, conquering the League isn't too big of a challenge in X and Y. D / P / Pl are my least favorite games in the series, but the one thing I did like about those games was that the Sinnoh League was actually challenging. Cynthia's Garchomp. That is all. I hope GameFreak makes more Champions like Cynthia in the future. She actually proved to be a decent challenge and Diantha was pretty underwhelming.
Like others have said, the story of X and Y is also pretty disappointing. B / W's story is by far the best in the main series. It really made you see the Pokémon world in a new light. X / Y on the other hand are different. Team Flare, like past evil organizations, was supposed to drive the plot forward, but they just seemed...there. I did like Lysandre, but I found him to be too similar to another crime syndicate leader (*coughCyruscough*). After Black and White, I expected future Pokémon games to have much richer storylines, but this was not delivered in X and Y. I can't help but feel that this part of the game was rushed a bit and I think GameFreak could have executed it much better.
How could they have improved the game to remove this disappointment?
Just add in a Hard Mode, honestly, or some mechanic that tracks your team's levels and adjusts the opposing trainer's levels accordingly.
What parts of the game do you enjoy?
Pokémon-Amie. The cuteness is off the charts. ^^
Also, I really love the new breeding mechanics. In the past, it was too time-consuming to try breeding for perfect mons, so I often turned to battle simulators for competitive purposes. Now, it is so much easier to get those perfect IVs and it is a change that I greatly appreciate. Thank you Destiny Knot!
Which Pokémon did you really enjoy using?
To me, it was both Fennekin and Skiddo. I normally don't pick the Fire starter, but I did this time around. Fennekin is easily one of, if not the, cutest starters ever. It is also pretty strong, making gym challenges take a lot less time, as only the Rock gym gives it trouble. Fennekin also put an end to the Fire / Fighting starters (about damn time GameFreak), becoming a Fire / Psychic type as it evolves. Being resistant to the new Fairy-type was also a nice addition and Fennekin is easily one of my favorites from the new gen.
I was upset that we couldn't ride the Skiddo that we catch, but it's not that big of a deal (yet again, that would be a pretty neat feature). The moment I saw a Skiddo, I knew I wanted to train one. Grass may not be all that great as a type, but Skiddo, along with its evolution Gogoat, surprised me. Unlike a lot of Grass-types, both of them are actually quite strong and have a good enough movepool to back it up, learning moves like Earthquake and Milk Drink as it levels up. Having access to Bulk Up was also pretty neat, and having such high HP made it all that much better. It may not have the best movepool ever, but what it does have is great for sweeping stupid NPCs. Overall, a pretty good Pokémon.
Which Pokémon ended up being a lot worse than you thought they would be?
Flabebe and its evos. I wanted to have a Fairy-type on my team and part of me really regrets not choosing Sylveon. Florges, Flabebe's final stage, may have high special defense, but that's all it has going for it. Despite that high special attack, my Florges could not OHKO a single opponent (unless they were Dragons) and often found herself dying more than my other Pokémon, even when she was still just a Flabebe. Her movepool also lacks good coverage, with Fairy, Normal, and Grass moves dominating her movepool. Delphox also shares this problem, but more often than not, Delphox actually carried its own weight, becoming a driving force of my team. Florges, well, just stood there, taking special attacks like a boss, but unable to dish out some serious pain of her own. It's a shame, too, because Florges looked like it had the potential to be a really good wall.
Which gyms did you find the most challenging?
None to be honest. Picking Fennekin really made battling through the gyms a breeze. ^^
These are just my two cents. I really found myself enjoying these games quite a bit, something that I haven't felt since R / S / E. The graphics are great, the music is as good as always, and the roster, while small, has some of the best-looking mons in recent memory. X and Y are simply "X-traordinary".