We visited Christiania a few years ago. It was lovely. The entire time, we felt just as safe as anywhere else in Copenhagen, and we didn’t see any evidence of drug use (certainly less than you’d see “on the wrong side of the tracks” in any other big city).
If you have a chance to visit Christiania, I recommend it. Copenhagen is one of my favorite towns. They have wonderful parks, museums, and even a really cool amusement park.