Allen Walker, after his master disappears, becomes an official exorcist to destroy the Akuma created by the Millennium Earl, which are created with the souls of the dead.
I personally don’t think it got very interesting until the Suman Dark’s Fallen arc at around episode 51-57. Until then, it seemed to go very slow. Afterwards however, it seemed to pick up and get much more interesting. I think the only thing that I didn’t like was that my favourite characters – Kanda (Sakurai Takahiro), Jasdero (Morikubo Showtaro) and Devit (Saiga Mitsuki) – get very little air time. I also thought that near the end it seemed to be adding in unnecessary flashbacks (yes, there were some where you got to learn more about the character past, but most were stuff you’ve seen already in the show).
I have to read the manga so I can give an actual opinion on this, but unless they come out with another season, I think it would probably be better to go with the manga. As soon as the anime starting getting really good, it ended.