Hi Jennifer!
We took my son to Disney last year---he turned 2 while we were there! So, yes! I say it is definitely worth bringing a 20 month old!
First of all, it's FREE. Kids under 3 don't pay for a ticket.
Second, he will love the colors, the lights, the sounds & more! My son asks me almost every week if we can go back to Disney World. He was already a big fan of Mickey Mouse Clubhouse so seeing Mickey and Donald and Goofy in real life? It was his unknown dream come true!
Yes, there is definitely the tired factor. My son refused to sleep in his stroller so we did a lot of early nights and took a day off in between parks to play at the pool and take naps. It's a different type of vacation when you go with a preschooler!
Some people think that it's not worth it because he won't remember it. My son might not remember it when he's 20, but he does remember it now. And my husband and I remember it. And it was a magical family vacation where we built tons of memories!
If you need some nitty gritty questions answered about traveling with kids under 3, there is a good page on Disney's site that has
lots of info.
I hope this helps!