Yup.. several times. I've turned into a Priceline maniac when it comes to hotel.
Bid 3 star downtown zone Miami, and you'll get Raddison, Doubletree, Hyatt or Sheraton. All in great locations, as far as proximity to the pier, and all decent hotel rooms. Start around $35 + .
4 star downtown will get you the InterContinental. We stayed there last year for $100.