Griezmann will definitely be a plus for Barca. He's technically gifted and he gives 100% on the pitch. He defends when asked to, something other attacking players don't do in Barca.
The only issue is there are now too many offensive options, especially if they go for Neymar. Valverde will have a tough time dealing with the rotation and it might cause some tensions in the locker room.

Speaking of Neymar, he just said in an interview that his best football memory was the 6-1 win with Barca over PSG. Let's just say that PSG fans are really pissed with him. He was transferred for 220 millions, brought literally nothing to the team except drama and now he is almost untransferable. Many people want PSG to make an example of him and do another Rabiot (leave him out of first team for a whole year). Of course, that will never happen but it shows how bad they want him out.

EDIT : there's a new problem with a possible transfer to Barca. The Spanish fiscal administration is asking for 35 millions from him.