I've lived in South Beach for several years now, and it is a world-class destination. There are, however, some misconceptions which should be cleared up: while South Beach DOES have many ritzy hotels, multi-million dollar homes, and celebrity visitors, it also has plenty of free and reasonably-priced attractions, and can be a fun and frugal destination for a traveler on a budget.

Let's break down the area into its component parts, from east to west:
1) the beach: free, wide, white, clean, with sparkling turquoise waters that are from 75-85 degrees F all year long, this wonderful asset is great for exercise, swimming, people-watching, and general r&r. It has a wide path along it inland of the dunes that runs from 5th street to 23rd street, where it turns into a boardwalk that runs for miles.
2) the Ocean Drive strip: featured on countless postcards, t-shirts, movies, etc., this street features mostly mediocre, overpriced food being hocked relentlessly by pretty young women hired to lure in tourists. The upside: even the bad meals come with a good view, and if you do your research, you can find the few places that are good!
3) Collins Ave: one block west of Ocean Drive, Collins features many boutique hotels, boutique shops, and a few solid restaurants. North of 15th Street, Collins is the street closest to the beach, and the hotels on the east side of it have beach access through their rear gates. The core of hot, trendy hotels runs along Collins from 17th to approximately 23rd Street (depending on which places you leave in or out), including The Delano, The Setai, the Shore Club, The W, The Raleigh, and others that fall in and out of favor. As you move further south, Collins is also the axis for most of the youth hostels on South Beach, where no-frills lodging provides fast friendships and vibrant social scenes.
4) Washington Ave: the 4-lane divided thorofare that is the spinal column of SoBe's club scene. Mansion, Crowbar, and an ever-changing menu of dance clubs that feature celebrity DJ's (live music is almost impossible to find in South Beach, and Miami in general) keep the music throbbing until last call at 5 AM.
5) Lincoln Road: an open-air shopping arcade that runs east-west between 16th and 17th streets, Lincoln is great for just walking around window-shopping and people-watching. Some of the restaurants are quite good (others are trading on their location), and if you can afford to buy at the ever-gentrifying stores there, so much the better.
6) The rest: from Washington Ave west to the water of Biscayne Bay, South Beach is a neighborhood, with people of all classes, backgrounds, ages, education, and beliefs sharing the sun-drenched mayhem. There is a whole sub-culture of Local Life, with a network of bars, restaurants, and clubs that the guy who is washing dishes at Joe's Stone Crab or busing tables at Prime 112 goes to when his shift ends, where he can have fun and knock back a hard-earned drink without breaking the bank. For those who bother to seek it out, this can be the most fun part of South Beach -- and certainly the most economical. A couple of great B&B's are also hidden in these neighborhoods.

The many attractions, hotels, restaurants, bars, clubs, and activities in South Beach each deserve (and have) their own reviews here-- look around for the things that suit your fancy. You're sure to find something you like.

Some tips:
Driving in South Beach is very frustrating. There are scads of roller skaters, skate boarders, cyclists, scooters, pedestrians, double-parked trucks and delivery vans, tour buses, and cars driven by lost tourists and locals on cell phones. Park your car at one of the city-run garages ($1/hour if you park before 7 PM) and walk around, or rent a bike.

Due to the significant number of non-U.S. visitors, most of the bars and restaurants include the tip. Check your bill to make sure you don't tip more (or less) than you intend to. They will remove it if you complain.

Service at almost every location in SoBe (even the fancy, expensive ones) is often slow and shoddy (the local managers call it "Beach Brain.") Don't fight it. You're on vacation. Relax.