Rio de Janeiro is where the traveler begins to feel that he's really in Brazil. It's not as tropical as the beaches up north but there's a strong party vibe and a good time can be had here if you know where to go.
Rio isn't the nightlife capital of the world as some of the consumer travel industry would have you believe but there are some cool street parties and a lot of kicking live music - if you're into samba and forro. There's a lot of cultural stuff going on, attitudes are modern and you can have a good time just hanging out on the beaches and in the bars.
Rio de Janeiro can be quite an edgy place to hang out though and pickpockets, muggers and scammers are always on the look out for dumb white tourists. If you go anywhere near the favelas you might catch a stray bullet from the firefights between the military police and the drug lords and, if you meet the regular police at night, they might try to shake you down for some bucks if they find any dope on you.
Once you learn the tricks of staying safe in Rio de Janerio, though, you'll be fine but taking all those taxis at night can wear a hole in your pocket and this isn't the cheapest place in the world to travel or live. The cost of living here can be around the same as in an average European or American city and if you're traveling on a budget then you're best off heading up to the beaches of north-east Brazil.
On the plus side, Rio is a great place to live if you want to study Bossanova, learn jujitsu or capoeira though. There are many good clubs and teachers and Rio is somewhere that's convenient and modern enough to be a long term place to hang around.
You can find enough educated people on your level to build a circle of friends and Rio can be a fascinating place to live once you understand the nuances of Brazilian society and you find your feet. At the very least you're unlikely to ever see so many hot girls and guys on a beach ever again.