Darmian is a decent player, I've seen enough of him, but I don't think he's much better than Clyne, we have done well for that position too.
Schneiderlin I rate very, very highly, wish we had gotten him. Depay will do very well as an individual threat and as part of the team, Firmino depends a bit more on the team.
They look to have a nice midfield group now though - Herrera, Schneiderlin, Blind, Schweinsteiger, Carrick.