Ive seen two, on seperate occassions, in my life.
One sighting was approximately 22 years ago on the Wreck of the Eagle in the Florida Keys (approx. 100' depth). That one was a big one, definitely in the 12'+ range. I remember there was excellent visibility that day and it did several passes on the wreck, each time slowly swiming off to the sand, only to eventually return again. If I had to guess, I would say it almost seemed interested in checking us out, because it returned a few times.
The second sighting was in approximately 1995 on a shallow reef in the Ft. Lauderdale area. This one was considerably smaller, maybe 6' to 7'. I remember being on the lower part of the reef, and seeing it swim over my head as it traveled the top of the reef in a Northerly direction. It never gave us the time of day.