If you havent bought/played Black or White, get into BW2, no need to play the first game, my missus didnt and loved BW2 (her first pokemon game).

If you didnt overly enjoy Black/White then you may not enjoy BW2. I thought they would be largely similar, but they are vastly different, the cities are more or less the same, but the gyms, routes and available pokemon are different. There are lots of legendaries from previous generations (and more if you get the dream radar) as well as pokemon that would be otherwise difficult to get in gen 5.

Overall I've enjoyed BW2, the Join Avenue and other online related additions are fantastic and refreshing, and the changes to routes and gyms are not cosmetic, I really enjoyed the challenges and new areas throughout the game (eg. BW2 start in a new area to the south west, while the original starting area in the south east is only accesible post game). You also get a proper pre-elite 4 training areas and a decent story, that is more fun and interesting than the original BW.

If you have time to squeeze in 50-60 hours before X and Y give it a go.