I actually used the Nintendo Wi-Fi Connection on the Wii and DS quite a lot. Some of my favourite memories include Tetris DS' item battle multiplayer (insanely fun) and Goldeneye 007's multiplayer on the Wii (in which I put in close to 100 hours). I loved Goldeneye's simplified version of COD's multiplayer. I also enjoyed the unique character names I would see online, like "Cereal Bowl" and "Sex Panther".
One of the big things on my "To Do" list before the service shuts down is to unlock the daily puzzles in Professor Layton and the Last Spectre. I would also like to download the various quests in DQ IX, but I imagine that I would have to play the game for a bit before being able to do that.