A few days ago I would have posted that you should definitely pick him up because he's been a Run machine, picking up 40 already. But since Runs are more of a product of who is hitting behind the batter than the batter himself, especially for a player hitting as poorly as Jhonny, it's much more of an iffy pick-up now. Instead of monsters like Martinez, Hafner, Broussard, and Blake hitting behind him, he'll have unremarkable people like Boone and Belliard to rely on.
Now, that said, the Cleveland lineup is still the juggernaut, only now Peralta will be hitting behind the monsters instead of in front. Therefore, there is potential here that he can transfer his absurdly high percentage of Runs for every hit he got to the same absurdly high percentage of RBIs for every hit.
If you want RBIs, I believe he'll be a steal for you. Just realize that your average will take a hit, his current Run pace will likely plummet, and you're not likely to see him hit 20 HR this year.