Personally I like it- gives a much-needed sense of direction in normally haphazard post-game Pokemon. I mean, in the original B/W, half the Unova region is left unexplored when you defeat Ghetsis, trainers with Pokemon 20+ levels higher than yours pop up, routes that, unless you read the walkthroughs are a pain to navigate, etc, etc. In short, you don't know what to do. Medals grant you certain (sometimes impossible) goals to work for, reset a few times in ragequit, and hopefully reach. They also give a sense of achievement, especially in the meaningless pursuit of Catching them All (which I am still working at) or the not so meaningless one of finding a Shiny (which I have actually succeeded in! Shiny Sandile on route 4 !).
The only annoying thing is that I keep imagining an 'Achievement Unlocked' banner materializing on the bottom of the screen when I receive a medal... too much console gaming, perhaps?