I haven't been, but previously did a lot of research before deciding not to go.
It appears to be easy enough to get from Miami to Managua, however, the "rub" seems to be getting from Managua to Big Corn Island. Recently, there were 2 puddle-jumper airlines, but one is no longer flying. Reservations for the puddle-jumper are a must (at least they were a couple of months ago), and if you miss your connecting flight out of Miami (or even if it's late), you could be in Managua for an extended period of time. I forget the name of it, but there is a hotel directly across the street from the Managua airport that caters to American tourists and business travelers. I think they charge about $85 per night. Also, the last I heard, the puddle-jumper airline was charging $1 per pound for all luggage in excess of 35 pounds. That was the clincher for me, I went to Costa Rica instead.
If you want to go from Big Corn to Little Corn Island, the only way to get there is a water taxi, and the ride on the pranga can be very rough. I don't think there are many places on Little Corn that have A/C, since most of the "hotels" don't even have electricity. There is only 1 dive operator (each) for Big Corn and Little Corn, and they don't give discounts for having your own gear. From what I've heard, the diving is "fair-to-good", but not spectacular.
I also found that the hotels were less than up-front concerning the amenities they offered. Don't assume that you'll get A/C, private bathroom, hot water, fan or even full-time electricity or an indoor shower.