I won't talk about the financial aspects, just him as a player.
Positives:
1) On form he can be a statistical monster, even if it is short bursts and purple patches. If he hits form he can really go through a scoring and assisting run;
2) His passing can be a bit better than people claim at times, he is capable of good long range ones and through balls, though he only showcases it rarely.
3) As people have said, playing with better players could get more out of him. Especially given Flick's system that is designed to maximise the output and abilities of forward players;
4) Could be a good use for FKs, as Barca don't really score them. Lamine wants to step up though too;
5) He seems to really want to be at Barcelona, which is a good sign. He does better when motivated and all the more so with it being a World Cup year;
Negatives (some of these will seem the mirror image to the pros):
1) While he can produce purple patches of form, he can also blow cold for long spells too, and is highly inconsistent. He's also the type who drifts through games not contributing much or at all in general play, even when he does score. Though I have some degree of Faith that Flick can make him a better all-round player, even temporarily;
2) despite some claims to the contrary, I don't feel he can play CF or RW. If he does play CF it would be more as a fluid front three, as he cannot hold the ball up like Prime Lewa could. He could do a job on the right when Lamine rests, but he's at his best cutting in and shooting or coming inside and dribbling, so it doesn't suit him as he is pretty one-footed in general;
3) While playing with better players can make one a better performer, it can also be that they do less well while not being one of the top men, as he was with fernandes at United in his best spell;
4) His attitude can be bad as everyone knows. If it turns sour he could well sulk at being Raphinha's backup in big CL games or Clasicos. This could destabilise the whole team. He's a huge focus of attention in the English media so any drama related to him will be huge news over here and in Spain, and exactly what Barca don't need in preparation for a big game, as it's already chaotic enough.
There's more, but just collating some of the things I have noted about him in the thread into one post.