My parents (late 50s) and I (31) cruised on the Diamond Princess to Mexico this past January. This was by far the best cruise experience we have ever had.
Pre-cruise: The cruise was preceded by a week-long trip to Las Vegas where my dad attended a convention, therefore we flew to LA from LV on Friday evening. We arranged to rent a car thru Hertz using the AAA website, and got a fabulous deal, and arranged to drop the car off in Lomita the next day, where they called us a taxi to take us to the pier -- they will credit your bill $15 for the taxi fare, which meant the bill came to around $3! We spent the night at the Best Western Los Angeles Worldport in Wilmington -- nothing fancy, but very clean, huge rooms (we had one with 2 king sized beds), and best of all for us right next door to a Denny's. We got a rate of $60+tax, which included a continental breakfast, thru Travelocity ($5 cheaper than the BW website).
Embarkation: We arrived at the pier at approx. 12:15. Had to wait a couple of minutes for a porter to take our luggage, then were handed health forms to fill out (Norwalk screening, if you prefer), then into the terminal and up to the check-in area. TIP: You are handed a number when you enter the terminal which determines the order by which you board, one per cabin; if you have to wait with your luggage as we did, have 1 person in your group enter without you and receive the number -- it's easy then to wait at the foot of the escalator for the rest of your group, and you have the earlier number. Check-in is where being Canadian really pays dividends -- big lineup for the express check-in, 3 people in line for the non-US citizen check-in! Of course, we only just got ahead of the Japanese group ... We were checked in in under 10 minutes, and had to wait maybe 20 minutes before we boarded -- very impressive (our number was in the 800's, by the way). Once on board, there were plenty of crew members point us in the direction of our cabin.