Yes I've been there at least once. :P
(look where I'm from beneath my avatar)
Check out Lincoln Road, lots of cool shops and great food; tons of hard bodies walking to and fro too. (Just keep your belonging within eye shot, and please don't talk to strangers!!) Also don't be shocked if you spot a celebrity or two walking the streets or taking in some rays on the beach.
Tip: If you're going to take a dip in the ocean stray far from Miami Beach! Tons of tourists will drive you mad. Go to where the residents (me!) go: Key Biscayne. Do not park on either the first or second key even if you see wall-to-wall (or palm-to-palm) bathers on either one. Go straight to the third key and you will land in good old Key Biscayne. Go passed the marina and watch for the signs leading you to a much better shore lines of Key Biscayne. The signs will point you to hang a left (east) as you drive south down the key. You may have to pay for parking but it's worth it. Being a 32-year old Miamian, you can trust me on this one.
But if your here to check out the action then Miami Beach (at night mostly) is a nice place to enjoy yourself. Hey guys! The women down here go topless on the beach so bring your cell phone cameras!!!
If you want to chill with a way too cool Cuban dude (me!), I am usually hanging on the mile in the Gables. That's my hood. I'm the turkey with the tats.
_