For those who haven't gotten X/Y yet, the new EXP SHARE item differs from previous incarnations in the following ways:
1. It's received extremely early in the game.
2. It distributes experience to your entire party, rather than just the one who held it.
3. It's not actually a held item anymore, therefore allowing the party members to hold actually useful items.
Basically, the archaic practice of having to constantly swap out pokemon to ensure that they're all getting experience has been massively mitigated. You can literally not use any pokemon except your starter and the rest of your party will not be the same level as that starter, but will at least not be in any danger of falling massively behind.
What I'm saying is that this is the greatest thing to happen ever.