Not safe by night . I was attacked in Copacabana beach after sunset by six young thief, which try stolen my camera, after short fight and shouting at loud they runaway without nothing. Some places are not safe also at day , like parks and slums even if you going with tourists guide you can be robbed. (even your cloths can be taken,but its kind of attraction:-) So better if you leave your valuables things in hotel.
On the weekend Copacabana and Ipanema are too busy and noisy. Empty in week days. Flamengo is really nice beach with nice view warmer and calm water. Long cue to the Jesus statue and Sugar Loaf. Too many people over there.
Overall Rio is worth to visit,but more like a city break then long stay holiday.
Best Things to visit in Rio - watch video:
Ipanema Beach Video
Rio in photo: