We did San Diego to Miami in 2003. If you want ot go, do it! Its brilliant. You can stand on the side of the ship and reach out to touch the lock walls. Thats how tight it is. Some lines have stopped doing full transits due to the fees, but Celebrity and P&O (at least) still do. You will find loads of info at
http://boards.cruisecritic.com/
We did it last year on Celebrities Equinox. as a day excursion. I was really bored as we didnt move very quickly. There was a few cargo ships going through at the same time and we stood in line for ages before our boat could go. Our timetable was very late.But being as we had booked a cruise trip that was no problem.
it's not all locks, most of it is rivers, lakes and other waterways that have been joined up. Look it up on Google Maps and you can zoom right in. It goes the opposite way from what you'd expect - the Caribbean end is in the west. Yes, it is kind of slowish. We just did it as a day excursion from one end, going to the other on the water then returning by bus. The views aren't from such a high position as on a ship, though.