I live in Ft. Lauderdale and work in Miami, been using ATT since the Cingular merger, and they have gotten better.
HSPA+ covered basically everywhere I am, and LTE covered in both cities, if I upgrade I will be fine....ATT does have a dropped call problem, but I wouldn't give up my unlimited data for it. I also call and surf the web at same time, I know verizon is getting it eventually, but doesn't have it now.
ATT isn't amazing, but it has me covered where I need, but if you want to jump ship, just know Verizon is basically the same thing, their just the political parties of the Cell phone world, but you could always test the waters and return if you aren't satasfied, gotta love returns.