I've completed three separate playthroughs of Diamond, and haven't picked it up since finishing the last one. I guess it's run its course.
My first playthrough started from when I got Diamond in 2007. I was a mere eight years old, and selected Piplup as my starter. This one's one of the most complete and in-depth playthroughs of any Pokemon game I have ever played: I managed, through some extraordinary determination, to get Piplup (by then Empoleon) to Level 100 (I've never gotten another Pokemon to Level 100) and obtained more than half of the Pokedex (no small feat for an 8-9 year old with no Internet access). I would periodically return to this playthrough until late-2011, when my save file corrupted owing to me failing to take the cartridge out of my pocket when my clothes were getting washed.
My second playthrough began in around December 2011 and ended around June 2012. I started out with Infernape, did a "speed" run through the game, barely ever even attempted to grind, and when I felt I had done enough, I transferred everything to my Black save file and erased my data.
My third playthrough was short and sweet; it started in September 2012 and ended in the same month. Once again, I ran through the game very quickly, trying to be as deviant in what I caught as I could from my last two playthroughs (this time, I started out with Turtwig), and as soon I had beaten the Elite Four, I transferred everything again, erased my data and put Diamond away in a comfortable place. I haven't touched it since.