For DS & I, a
Disney cruise is the best of both worlds. It's pretty darn cool to be on a lift and have Goofy or Snow White or somebody just hop in there with you. Like a mini private character meet & greet.

And the Magic is like home-away-from-home for us.
That said, they have been priced smooth out of my budget for a few years now. But we do miss the Magic very much!! I'm thrilled beyond all mortal expression that she will be sailing from so close to my house next year, and I am hoping to be able to save up enough to be able to sail her again by early 2013, but we'll see. It really is a huge difference between
DCL and other lines. HUGE. And it's a huge difference between a WDW vacation and a DCL cruise, especially since there are so many ways to save on a WDW vacation.
Cruising itself is the only vacation I ever take where I don't feel like I need another vacation afterward to recuperate from the first one. DS & I have sailed on DCL, Princess, & RCI, and are booked on RCI again for next February. Princess was the only one I wouldn't do again, because their kids' program was so abysmal. But from what I understand now, it really varies from ship to ship with them. He and his best friend had *such* a great time last year on RCI's Voyager of the Seas (they were in the teen program by that point) and are looking forward to just as much fun next year on Mariner of the Seas.
The other main difference for me between cruising and WDW, aside from being able to relax, is that on a cruise I also get to be a grownup.

I've been a single parent since DS was under 2 (he's 15 now) and there haven't been many times when I've gotten to do grownup things. Don't get me wrong -- we LOVE our WDW vacations, and I cherish every single memory we've made there. And always looking forward to more! But on a cruise, there is always so much to do for every age group, so many activities to choose from, he gets to do his own thing and I get to do my own thing and we also meet up to do stuff together. It's just ideal to me.
And cruising is easier to budget for, to me, because all the necessities are included and I only have to factor in "wants" like photos, shore excursions, bingo, etc. But you can spend precious little onboard if that's what you wanted to do, knowing that your accomodations, entertainment, and more food than you could possibly ever eat is all included.
We usually alternate between WDW one year and a cruise the next. It makes the anticipation for each trip that much greater, and keeps things fresh. We just spent 8 nights in WDW in February, now can't wait for our next cruise!!
