I wasn't keen on him being bought to be honest. He was just another name to fill in the mysterious number 10 role that United felt they had to bring in, and everyone they have brought in to play that role has failed and that includes during Fergie's time at the club. He's capable of being sublime and running the game for us, though hardly ever these days. He was never blessed with speed, so when he started out wide he was always going to struggle and his desire to track back when out wide is questionable.
However, somewhere in the midst of all of this successive managers have constantly selected him in squads and continue to play him, and yes at times he's invisible for long periods he's liable to deliver something for you. Would I say he's a flop? I'm not sure flop is the word I use I think he's been a bit of a let down for us, never really stepping up conistently for us as he had at Chelsea, but then we've never played a system suitable for a "number 10" to shine and we've had some good ones who have tried to play that role for us and fail. We've let him down in that sense, but he's adapted the best he can to our style of play and does a job for us when called upon, which is what we ask of our players anyway.