If you are to drive (or take a bus) to both ends of the island (North and South) be ready for a long, but I mean long, wait in the most intense bottle-necks I hade ever seen in my life. Summer weekends, specially after 5 pm, are absolutely impossible in Florianopolis. If you have any appointment downtown (or any place for the matter) before 9pm, and you leave one of the south or north most beaches at around 5pm, forget your appointment and be ready to be stucked in heavy and slow traffic for hours. So, my suggestion is that you leave early or just take your time for extra caipirinhas (have someone driving for you if you drink more than one) and stay on the beach until things calm down on the roads. The only good thing about being stucked is that people leave their cars and dance, chat and drink meanwhile the traffic stands still. Well, this maybe a great option to meet real nice people.
Florianopolis is still a safe place, not as much as it used to be but with a little precaution you can have a great experience without surprises. One of them is not to walk downtown late at night, especially after the stores close 6-7 PM.