I'm just baffled by all the people here saying oh yeah, you have to go when the kids are super young or the magic won't be there for them. I can tell you I did not go to Disneyland (still haven't made it to DisneyWorld yet) until I was in my early teens and it was super magical and I loved every second of it. Still feel that way every time I go.
Yeah I was starting to feel a bit foolish too - I first went at 12 and loved every second of it, maybe some posters think I was a stunted kid or something huh?
My stepmom first went in her 50's and had such an amazing time she and my dad have been back 3 times more... but I guess there must have been no magic for her either though since she was over 6 
Folks, if you want to go, go. But don't fool yourself that its only special for super little kids, Disney is world famous for, and prides itself for being amazing fun for all ages.
A little O/T but some history might be useful in understanding why the Disney parks aren't just for little kids. He set out to build a park that
he would want to visit as an adult. The parks are successful because they appeal to a broad range of people.
Here's a quote from the man himself:
It started with my taking my two kids around to the zoos and parks. While they were on the merry-go-round riding 40 times or something, I'd be sitting there trying to figure out what you could do that would be more imaginative. Then when I built the new studio in Burbank, I got the idea for a three-dimensional thing that people could actually come and visit. I felt that there should be something built where the parents and the children could have fun together.
I've been to the parks with adults who have never gone before -- cynical engineers, no less -- and seen the wonder in their eyes just like a 6yo's. I'm looking forward to going with my son in law and granddaughters; he's moderately anti-Disney, but I think he's in for a real surprise.
Mrs.k2002 and I have annual passes for Disneyland. Sometimes we'll go with our teenaged sons and sometimes just ourselves. Being 55 instead of 5.5 doesn't take away from the experience. I say that as someone who has been going since he was 3yo.
Back on topic: A parent
should be concerned when their adult child is making a questionable decision. There are limits to what they can do about that concern though. One way is to express it, but that has to be done in a way that respects the child's adulthood and autonomy. "How will this affect your plans for buying a house?" raises the issue, and addresses what should be the parent's real concern: Did the adult child consider enough factors in making this decision? What we can't do as parents is impose our particular idea of what something is worth. For many people, house over Disney is a no-brainer. For others, it's the opposite. Neither is really wrong. It's important that the decision maker consider the value of both before deciding.
Could a Disney-over-house decision be the symptom of real irresponsibility? It certainly can, but that's not in any way a given. Treating it that way at first is an insult to the kids and an insult to the parent (if they had raised the kids right, they wouldn't be irresponsible.) The FIL in the OP sounds like he's going right to the "you're being irresponsible" assumption.