We recently completed a transatlantic on Celebrity's Constellation from Miami, Florida to Hamburg, Germany, and could not be happier with our experience. After disembarking, we stayed on in Germany for 10 days, using rail passes to travel through the country. We only got home a week ago, and are already searching for another transatlantic cruise for later this year -- that's how much we enjoyed it. But, I will start at the beginning:
Pre-cruise boarding: We started in Naples, Florida, so we rented a car one-way and drove it to the Miami International Airport and dropped it off. Then we took a cab to the port ($24 set fee), so getting to the port was a breeze. This saved us a lot of money and worry, as we did not have to leave our car parked somewhere for so long.
Embarkation: A breeze. We arrived around 1:00 p.m., and dropped our bags at the curb. Even though we had not received our bag tags, we filled them out with the porter, attached them to our bags, and in a matter of minutes we were headed inside. (This was a different experience than when we left out of Miami on a prior cruise with Carnival.) There was no line to wait to go through the scanners, and not many people in front of us when we completed our pre-boarding paperwork. I should note, that we carried on 2 bottles of wine (as permitted on Celebrity's website, and saw a number of other passenger do so as well) with no problem.