On par, if not better than HeartGold and SoulSilver
I played Pokemon Black when it came out in Japan in September of last year and I've got to say I loved it. I was worried that it wouldn't live up to expectations and be as good as HG/SS, but frankly it did not disappoint. The new generation is fresh and is a completely new start for gamers; containing a world far away from the older generations. It was greatly needed as it helps new gamers immerse themselves in the world of Pokemon, but retains its charm and keeps its sense of identity for the older player to enjoy. My only real worry is that the gyms in this generation are much more linear than they have been for the past four; however this does greatly help the younger audience find their way (in previous games I have found that younger players tend to get frustrated when they have to train and backtrack to older cities to face gym leaders).
It's not just the story that has changed: 3D has been improved from the last generation and a more dynamic camera has been included which, in my opinion, give a better sense of a new world to the players. New battle techniques such as triple battles and rotation battles are great addition and add a new stragegic element to the games. All of these additions make these games the most promising yet, and hopefully they will not disappoint.