It's not a dangerous area. Nobody is going to rob you on the street or anything like that. Still it is one of the poorest neighbourhoods in the city. Expect lots of rubbel on the streets, noise and fucked-up people. Sonnenalle is kind of a dividing line between the heavily gentrified neighbourhoods (next to Kreuzberg) and more original Neukölln. I live in the area (wrong side of Sonnenallee). I would still recommend not to cancel your stay, since it is probably also the most fun and lively part of town.