I used to hate it. Now I'm on the fence. For Pokemon you've caught yourself, it's very helpful in leveling everyone at the same time (except sometimes, things overlevel - my Espeon was Level 85 whereas everyone else was high 70s).
The difficulty I'm finding is that in the beginning of the game, upwards of Level 30, there aren't enough badges to permit using traded Pokemon. Without the third badge, my Scyther, Vivillon and Larvesta were on the verge of not listening to me. I had to therefore turn it off, else box them if I was leveling a different Pokemon. (At that time, I just bred an Adamant Venipede and needed EXP share on to level it from Level 1 to Level 30+). If the game introduced the second and third badges sooner, it wouldn't be such an issue; however, you're very limited at the beginning if your Pokemon happen to be traded. It's only useful again after the fourth badge where Pokemon up to Level 60 obey. That's a huge frame considering the gym is Level 35ish and my Pokemon were low-end 40s at the time. I don't think going forward I should have any further issues with it.
I'm also not so sure about how leveling is now. I've never managed to get Pokemon to Level 100 before (I'd always stop at end-game, around Level 50-70). It's a bit shocking to see how easy it was; even unrewarding.