Disney considers us to have sailed in the inaugural "year" even though we didn't sail until 1999. I believe that the day after we completed our first sailing, the Wonder started service, or at least arrived at PC for the first time. And yes I've sailed on all 4 ships, plus the Re-Imagined Magic. And I've been on the DIS for a long time (just the Cruise board). I see from my sig that I joined the DIS between my Eastern and Western cruises.
We sailed in the first year of the Magic going on the eastern, then western caribbean cruise. And we sailed through the Panama Canal on the Magic after she visted
Disneyland for her 50th anniversary. And we were in the Med the the Magic the first season she was there.
Didn't sail on the Wonder until 2011 when she came out west and headed to Vancouver, then on to Alaska
We were on the 4th sailing of the Fantasy
finally in 2013 we sailed on the Dream.
There are a group of people who have sailed on many "first" cruises - of the ship or on new itineraries. Generally speaking those are the people you want to avoid, they have the "its all about me" attitude. How do I know, I've seen them with their tee shirts and acting poorly to other guests and the crew.
And yes, I've been on many of those cruises or at least on "new" cruises - since we didn't know if
DCL would return (especially back when there were only 2 ships). As it turned out the Eastern and Western Caribbean cruises turned out to be popular, as did Alaska and the Med. Frozen reinvigorated the Northern Europe cruises (and not charging an arm, leg, eye and a kidney helped as well).