So I'm up to the second gym (10 hours or so of play, I must move slowly...) and my team has an average of lvl 25. I got the event Torchic which has evolved in Combusken... but it has steamed ahead to lvl 27 despite being my least used Pokemon. It's getting the bonus EXP because it's technically traded. Most trainers are around 10-15, people in the gym are 15-20 max... tonight I'll fight the gym leader, but I know it'll be a piece of piss.
I hate making games artificially hard. I know you can turn off the EXP share, but I have this weird compulsion where I play games based on what the defaults are. Like if a real-time strategy RPG doesn't pause in menus by default I'll usually play it that way. I know the EXP share is nothing new... but in Red/Blue it told you everyone's EXP making it too annoying to use and came late in the game, in other versions it was a held item and that the holding Pokemon had to be in the battle to make it work. The new one makes the game feel too easy.
I think if you like having a strong team, leave it on. I can certainly see the appeal. Even my seldom used bastards are strong enough I can rely on them. But if you want a classic Pokemon challenge I recommend turning it off... it feels weird at first, but I the extra challenge is worth it.
To balance the EXP share maybe they should have given it a draw back, like you get less money or less EXP overall... but I digress...