We just did this cruise on May 9, 2004 from Ensenada to Honolulu. We bought bus transfers from Celebrity for the approx. 2 hour ride to Ensenada. The buses were not what I would call luxury buses but they were adequate. The ride does go by some very nice scenery along the Mexican coast.
Luggage was taken from us at the airport and sent by truck first to the San Diego port for screening by the TSA people, then sealed into trucks and transported to Ensenada.
Check-in for the ship was done at the Hotel Coral in Ensenada. We sat in the buses for at least an hour there waiting our turn to move up to the room in the hotel being used for check-in. There just is not enough room inside for everyone. Once you have checked in you reboard a bus for the 5 to 10 minute ride to the ship.
On the 9 day cruise there were 2 formal nights, the 2nd night (1st day at sea) and the night we left Maui. There were more kids on board than I would have expected in early May and appeared to be a fair number of teenagers.
If you have any other questions feel free to ask and I'll try to respond.