I finished the main storyline, but I don't think I ever took advantage of all that the postgame had to offer. I think I visited all of the postgame areas, but I never finished my Pokedex, bred anything decent, etc.
BW2 are worse though. I still haven't finished the main game on those (think I have 7ish badges on White2? Can't quite remember--I was playing it a bit before X and Y came out, but I haven't picked it up in a month or so).
I'm kind of hit-or-miss when it comes to finishing the games. I used to play through fairly quickly, but now I tend to take my time more. In general, I tend to finish the even generations much more quickly than the odd ones (still haven't finished LeafGreen due to my obsessive and somewhat bizarre training scheme, and haven't done as much with BW as I would have liked, yet have a completed Pokedex and lots of bred Pokémon on Diamond).