A certain scientific accelerator... Isn't accelerator the main (and a damn good) reason to watch this masterpiece?
besides him (and his badass laugh), the animation is great, not to say masterful, the chara design is just so great and the soundtracks are really good (it depends). This sequel is a real masterpiece that everyone should watch.
Side Note: watch the original series "a certain magical index" to understand more. But you can also read articles about "index" main events to understand better the world of "accelarator". It goes also for "railgun", which i'll write a review about later.
overall i give it a 10/10, i wish it had a second and a third season like "railgun" and "index" did