We got the backpack style for DS for our WDW trip. We started using it in town so that he would be used to it when we really needed it in the parks.
DS is usually quite content to ride in the stroller, and we had our nice comfy one with us so it wasn't a huge issue, but it was very nice to know when we were in lines that he couldn't dart away from us. (He gets impatient being held for long - and it's so darn hot there we couldn't take holding him either.)
Please, get one a month or two before your trip and start using it - trips to the mall, a walk to the park (and then let your DC play without it, of course), wherever it is convenient. Not that any of us want to compare it to a dog's leash, but seriously, does a puppy instantly know how to walk on a leash and not fight it? NO! Our children, with more brains and more will (or stubbornness - LOL) are the same way. They need to learn to 'walk on the leash' the same way. Fortunately, since they are smarter than a dog, they get it pretty easily most of the time.
I do like the puppy/monkey/bear backpack styles (Got ours at Target for under $10) because they don't have the dog leash look. I remember my sister had a leather one for my nephew when we were on a ferry in NYC when I was a kid. It DID look like a dog leash and harness. But even then, I (1st grade) realized how much safer my nephew was and that I would probably use one for my child(ren) one day.
We got more "That's SO cute! Where'd you get it?" comments than nasty glares - both at home and at WDW.