Just our experience after 50+ cruises:
If you want to be one of the first people on the ship, check in on-line just after midnight the first day that check in is available - you can probably get the "10:30 AM" slot. You'll arrive with hundreds of other people (SOME of whom have the earliest check in times, some of whom are trying to beat the system...). Probably wait outside the terminal for a while, then wait in the X-ray line, then wait in the line to check in at the counter, then wait in the "holding area" until your group is called, then you can walk onto the ship (and wait another hour or two on the ship until your cabin is ready). I'm not criticizing this system, just sharing our observations having done it many times. These days, unless you are a suite guest, you'll simply wait with everyone else who has the same boarding time. You get a few hours of "ship time" ahead of the the people with the later check in times. (Of course some things like the stores, the casino, etc. aren't open until the ship sets sail in the early evening...)
For various reasons, we have also had the "1:30" (or later) check in time. No lines - no waiting. Walked right into the terminal, walked right through X-ray, walked up to the (empty) counter and walked right onto the ship (and right to our cabin). There is something to be said for waiting a couple of hours for the crowds to clear - particularly if the cruise is longer than just a few days and a few hours of added "ship time" really doesn't make much of a difference. I'd say that if you're not a fan of crowds (and waiting), just arrive in the early afternoon - even if you have an earlier check in time.
~Bob
