Good signing for them.
Does he improve their team? Yes.
Is he a good player? Yes.
Does he have high potential? Yes.
They'll get battered by us & other opposition fans for how they've goofed with him, letting him go for free then paying a world record to get him back - but if he helps win them trophies then doubt they'll care much. If we'd signed someone of that ilk (e.g. Kroos) for similar money we'd (most at least) would be happy.
Saying that I don't see it as a hugely worrying signing, he's very good, but he's not close to a Bale, Ronaldo, Neymar, Messi type signing. He won't win them games on his own. He's not worth £100m+ & he's not better than Yaya was for us (don't remember us winning everything yet we had a better player at CM..).
They're amassing a good team, but so are we & other sides. Lets see how it all plays out before making assumptions.