As a Dutchman and Ajax-supporter, my opinion is that Davy isn't brilliant and that actually very few Ajax-fans think he is brilliant. He is just a humble, hardworking and rather intelligent lad whose main strengths are his mentality, stamina, his runs inside the box and his finishing abilities. His leadership is often quoted, but keep in mind that this concerns his situation at Ajax with mostly guys who are below 22 years of age. I don't expect him to boss Rooney around anytime soon.
Davy always has had a lot of critics, even at Ajax. Yes, he often manages to put his name on the scoresheet, but his contribution to the team play was often negligible. He does put in effort to recover the ball but further prefers to be the end station of attack, rather than creating chances. He is often mocked as "Siem 2.0", of course referring to Siem de Jong for their comparable styles of play. I prefer Klaassen, but not by that wide a margin.
I like Davy, but I think it will be hard to become a star at Everton with his abilites. A useful player might still be a possibility.