I just completed the Delta Episode. The Delta Episode is reason enough to buy the game -- it's absolutely phenomenal and is everything Pokemon should be. An interesting story, with excellent music, good characters, and great cut-scenes. There's even a cute little teenage-like romance. Midway through it gets a little tedious with the constant bouncing around the region and all the battles but the beginning and ending are worth it.
Man, if this game had the Battle Frontier it'd be the best Pokemon game ever made. If they make Battle Frontier DLC then I hope another 'episode' is included -- maybe centered around another 3rd gen legendary like Jirachi?
(SPOILERS) Ending cut-scene of the Delta Episode -- https://www.youtube.com/watch?v=7X6yrHpYRd4 (SPOILERS)
The editing, camer angles, music, and timing in the ending cut-scene are something I never thought I'd see in a Pokemon game.
I found a video of just the music from the scene, so there are no spoilers. https://www.youtube.com/watch?v=Q0GMPne5PrM
"On my business card I am a corporate president. In my mind I am a game developer. But in my heart I am a gamer." - Satoru Iwata
