We did a land and sea cruise for our honeymoon back in 2009. We did 4 days in the parks (where we crammed in every minute from park opening til closing) and then 3 days on the ship. If you decide to tack a visit to the parks onto the cruise- I would recommend doing the parks first, THEN the cruise. The Cruise is much more relaxing when it is the last leg of the journey (and also, it's kind of the rest that you wish you had after a vacation anyway! lol).
To directly answer your question though, I will say that it was my DH's first encounter with both WDW and DCL - and afterwards, he decided for our next trip that he'd rather spend time in the parks, instead of going on another cruise with DCL. I wouldn't say that we MISSED WDW on the boat- but if given the choice between the two, we'd opt for WDW every time.
I guess it really depends on what expectations you go in with- if you're expecting exactly WDW on a boat- it won't measure up. The ship is not crawling with characters in costume for autographs (although they are there sometimes), you can't ride on Soarin or the Haunted Mansion, and you are not constantly bombarded with Disney tunes. If you look forward to a quality cruise, with great entertainment (make sure to see the shows!!), food, service and a spectacular relaxing getaway at Castaway Cay- then you're in for a treat!