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Exp. Share is useful, but it spreads the exp a little too thin sometimes. It's a big improvement over Exp.all, though. Using it in combination with the old Pokemon switch trick yields good results for me.
EXP.Share is a handy little item in the game. It definitely helps with training Pokemon up to certain levels. It's also nice to go along with EV training too... since all EXP gained gives EVs.
It's a very useful item. I have 2. One from my previous game and one from my current game. I use it to train pokemon upto a certian level or help them get trained to Lv. 100. example: my Raikou. I used Exp.share for him and it worked. He was getting almost 5000xp per battle.
Honestly, I hate it. It's only useful when you don't trade in for local pokemon, for example trading in your rattata for a Mankey or something, due to that honestly retarded belief that you care for your pokemon, it is however useful when EV training, nothing more.
I use it mainly for leveling up Pokemon that are weak in either stats, movepool, or both. I never use it for EV training. Macho Brace gets the job done better.
It's useful enough, particularly when I'm trying to evolve a Pokemon that I only intend on getting to evolve to fill up my Pokedex or for Pokemon that I want to evolve but really can't battle if I tried to get them to (like Metapod).