The Oasis class ships are amazing. We've done Oasis once, and would do it again (or Allure).
Fact is there is so much to do onboard that these offer more than any other ship for food, entertainment, and activities.
As for being crowded - we never found that to be the case - the bigger the ship, the more room for cruisers to get around and do things.
There's no doubt that when you've got 6,000+ passengers...that reflects alot of people on board. Then again, we're talking a ship with 18 decks, 1187 feet long and 208 feet wide - alot of room to move about without problems. Add 23 restaurants (not even including the gigantic dining room), Aquatheater with shows, 2 flowriders, minigolf, a huge theater for shows, ice skating and skating show, pubs, shops, promenade the size of a mall...and so on....this should be on every cruisers "have to do it at least once" list.
We'll go back on Oasis or Allure sometime again without any hesitation.