He's a quality player. A classic Spanish CM that's great at the signature Spanish 'pass and move' game with good vision. What's better for United is that he's well suited for the EPL given his height (182cm) and his extreme intensity as a player. His defensive workrate is outstanding and he has great stamina. He's similar to Koke in style. His flaw I'd say is that he's a bit 'rough around the edges' in action but I don't see that being an obstacle given that he'll be playing in more space in the EPL, under a coach as seasoned as Van Gaal, and in a slightly deeper role (CM rather than CAM where he played under Valverde).
As for his cost, it's certainly over the top with inflation and taxes. As of right now, the price is too much for an initial investment but he's fully capable of delivering and being pivotal in United's ascent back to the top. He's expensive but he can prove to be worth it.
Have a look for yourself, this channel has made a decent amount of game highlight vids for him:
