TL;DR - Only buy if you are want to get into the competitive scene of the Pokemon games. Barely any end-game, non-memorable characters or story. Easier than any other Pokemon game.
Why? There are no end-game objectives. At the end of the Elite 4, you are going to spend most of your end game probably catching up on your Kalos (the region) Pokedex, only to achieve a ribbon that makes the amount of eggs you would receive from the Daycare more frequent. There is probably only one real end-game thing you could do and that's the Battle Maison. Like the Pokemon Tower from previous games (Gen 2,3,4, etc), you are pitted into a 3v3 endurance test against the AI, who get significantly stronger as you progress.
What about the competitive scene of Pokemon now? Well if you're new to it, let me sum it up as a thorough as I can. There are secret values a Pokemon can have that you can give to it that can make it stronger than ones that don't have those values. We call them EV's and IV's. You can easily give EV's to your Pokemon through the Super Training app on the touch pad, which is much faster than older generations where you had to find a specific Pokemon to get that value from and the minimum of that would be 1. With Super Training , minimum is 4. The max EV in one stat can only be 252. IV's are different. These are more
hidden, however can be viewed by a guy in the last town you visit after you defeat the Elite 4. Not only did Gamefreak add an easier way to see them, but also to obtain them. If you give a Pokemon in the Daycare a specific item, it will pass down it's IV's to the child Pokemon. There are two of these items in the game, so you can easily pass down a perfect 6 IV Pokemon in probably less than 12 hours if you're lucky with the RNG (Random Number Generator).
Why is this important? EV's and IV's can pretty much give your Pokemon 64+ additional stats in the stat you want your EV's in. If you are facing a Pokemon without EV's or IV's then you are a significant advantage.
What else is there to do? Friend Safari? Why yes, Friend Safari. Friend Safari is like your own practical Safari Zone that you mooch off your friends that share your Friend Code. Each Friend Safari is based on the person's Friend Code and that defines what Type of Pokemon you get in the Safari, as long as three significant Pokemon. However, the third Pokemon can only be seen if both you and your friend have been online at least once at the same time.
Right, now onto the less important stuff, but however, still important overall. The story and actual pre-Elite 4 gameplay.
Trying to not to spoil the entire story or ending, you are helping a Professor to complete the Pokedex just like in all the other games. You receive your new regional starter and you're off onto your adventure! But wait, there's more!
If you picked up your game recently (I don't know exactly when the event ends or if it has ended), then if you go to Mystery Gift then you receive a special Speed Boost abilitied Torchic. If you missed the event, then you're in luck with even more surprises!
Probably about 20 minutes into the game, you get an item called Exp. Share. If you remember from
previous games, the Exp. Share gave the Pokemon that held the item extra Experience even if it wasn't in battle. This time it's different. Exp. Share is now a Key Item that can be turned on or off to your liking, however if it is kept on, the Pokemon in your party will now gain the same amount of Experience as the Pokemon that had battled would have gained Experience. Exciting. Not really. Let's call it easy.
Want more? Okay, well when you reach the main city, you receive a new Pokemon. A Generation One Pokemon. Yes, like Squirtle, Bulbasaur, and Charmander. Isn't this fun?? Now without going further to which ones you get, there are also two more Pokemon you receive. For free. As in freedom or something like that. The game goes by just so fast when you have 3-4 Pokemon you didn't even have to waste a Pokeball on and get free Experience. In fact, so fast that you will probably not even remember the story, the gym leaders, or even the fact that the new legendaries aren't even that great as you sit and moan about how long, tedious, and boring the story can get because you have Pokemon that can one-shot every opponent the game throws at you granted you have the Type-advantage, which at this point you should. You have 3-4 Pokemon that are given for free that basically cover each others weaknesses.
You just want to defeat the Elite 4, catch all the Pokemon, and retire as a Pokemon Master, but all this damn story is getting in the way and you'll get tired of it. Sure, the gyms are different and nicely designed, but really, the gyms are lackluster. The Elite 4 is lackluster. The champion of the Elite 4 is lackluster. What are you really gaining out of this? Really, what are you gaining? The achievement of knowing you beat one of the least memorable and possibly worst Pokemon game Gamefreak has put out? There are only 69 new Pokemon in this Generation. Only a few are worth mentioning that don't look horrendous or atrocious in design.
Let me point out a new feature also that ties in with the story. Mega Evolutions. If you Pokemon is holding a specific item in battle, you get the chance to Mega Evolve it. Not all Pokemon get this, just some, mostly Generation 1 Pokemon, even then is about 10-15 overall. Some Mega Evolutions give better abilities, base stats and even change their Typings. Let's take Gyrados for example. Gyrados is Water/Flying. When he Mega Evolves, he now becomes Water/Dark. This removes his 4x Electric weakness into a 2x Electric Weakness. Not really useful, but the bonus stats could prove useful later. But geez Gamefreak. The designs. The god damn designs you made. If it wasn't the fact that you eventually become used to their awful design, then I would have never picked it up again after I finished it.
What's left to talk about? Let's go back to end-game. If you are going into competitive Pokemon, you will be sitting your ass off for 6-12 hours a day, granted you have that much time, to fully EV, to get a 5/6 IV, to train a level 50 or level 100 Pokemon. Most likely one or two a day. It's boring and tiring and sometimes I think it's not even worth it only to be put into a 3v3 battle on the GTS. Where's my 6v6? Oh yeah, you can only 6v6 your friends via Friend Codes. Maybe I should have put it in earlier, but when you go to the Homescreen of your 3DS to input a Friend Code and back onto Pokemon X/Y, you will be disconnected from the internet. Rude, I know. Tedious, I know. I'll say it again, sometimes I think it's not even worth doing anything at the end of the day.