He is what he is. He is a very intelligent player, good at free kicks, can make intelligent runs, and can pick a great pass in the right areas.
There is two points with Juan, why he will never be recognised as a great success at a club like Utd.
Firstly, as supporters we have been brought up with teams traditionally having pace, power and penetration, Juan has none of these attributes, he doesn't have a turn of pace and doesn't go past players, if he is it out wide, he cant give you what Giggs or Beckham gave you going forward or defensively or if playing behind the strikers centrally, he cant give you what Rooney could give you. So he will always be compared (and rightly so because they are standards) with players that are more effective in their own ways.
Secondly, in the bigger games, you always felt that defensively you were playing with 10 men with Mata was in the team. He works hard don't get me wrong but isn't effective at that side of game at all.
Has he been a flop - I would say No that would be a bit harsh. Has he made enough of a difference - I would say No.