I actually never bothered with experience share that much, where I did with the previous games. I used it until my Bulbasaur and Mystery Gift Torchic evolved into Ivysaur and Combusken, just to help level them closer to Froakie, but never turned it on after that. It seemed too good.
Yea my issue was that I never realized how good it was until basically the 5th gym. I was used to gen V, where even using boosting items you are just at the level of the E4 and gym leaders. I also didn't realize that it doesn't split the exp anymore, but actually multiplies it out such that you earn 350% normal XP with it on (main poke gets full XP, other 5 get 50% of that). I knew I was destroying the game but it felt too late to turn it off by the 5th gym, which is why I started over. Let me experience new pokemon anyway I guess.
That said, I still ended the game with a level 80 and a bunch of mid 60s, but I like that. I always felt like mid 50s-60s was too low to end the game, not that much was necessary after. Though I guess they did it to avoid having pokemon learn moves that late, and to avoid you levelling up with no feeling of reward for a while (no new moves).
I've been wanting to try replaying the main story again to try different runs, but I don't want to reset and lose all of my items and TMs and post-game stuff since I'm participating in the League. So I'm thinking about buying a fresh copy of X while I keep my post-game progress on Y. If I do, will my FC apply to both X and Y carts? Will X automatically populate the in-game friends list with my system-level friends list?
The FC applies to both, it is all system level. My gf plays Y on my 3DS while I play X. Though my case is a digital copy of X physical copy of Y, not sure if any other setup would change it. You can also still trade yourself through the GTS with only one 3DS with some clever finagling.