I think this movie was okay. It isn't Hitman: Agent 47, but it isn't Sonic The Hedgehog. I think it's a competent DC animated movie, but a bad adaptation of Batman Arkham Asylum. I'll explain why I feel mixed about this movie. Starting with the good.
The voice acting is GREAT. Every DC animated movie has awesome voicework. If you've heard Superman/Batman: Public Enemies, you've heard Assault on Arkham. Arkham Island and the characters that appear in the games were faithfully animated. The Suicide Squad were also interesting, but they are also why I think the movie falls into the videogame movie curse:
They are the main characters. Not part of the main cast, they are the main attraction. The plot is that Amanda Waller gives the Suicide Squad the mission to kill the Riddler, which changes his role as a side quest, but because of the focus on THAT story, the whole Arkham takeover is sloppily shoved into the 3rd act. I like the callbacks to the Detective Mode and Explosive Gel, but I really was expecting a Batman Arkham Asylum movie, with the Suicide Squad, although TheMetalBlade5 said it was a Suicide Squad movie.
Although I have a love/hate relationship with Batman: Assault on Arkham, I would recommend you go watch it, it's really fun. Though don't expect a full-on Arkham Asylum movie.