We got one when my daughter (now 19) was in first grade... under duress. My daughter hinted about it a few several times and I was just like "Shrug. I don't know how you get one, honey. But it's OK, we don't need one." I had heard about the gist, but I didn't realize there was an actual Elf on a Shelf. One night, she was crying and I asked why. She said "I asked my friend who has one and she said all you have to do is wish for one. But I've been wishing and wishing and he still hasn't come!"
Awww... sigh.... OK. So I bought an elf doll from the Hallmark store (not an official Elf on a Shelf -- like I said, I thought it was a "concept" not an actual product -- but she didn't seem to care about that. Ours is way cuter.)
She was so excited the morning he arrived and demanded "You should all thank me because *I'm* the one who wished for him." Sure, Dear. The rest of us will be lining up to thank you for this!

Then she said "Wait. Is there only one? I wished for two!" I did NOT get a second one.
She "believed in him" for several years even though he pretty much just moved around day to day (very few funny antics). One time, we almost caught the elf on fire by positioning too close to a lightbulb. DH flung the smoldering elf to the floor and my daughter was in near-hysterics... but it wasn't because the elf was melting. It was because Daddy had touched it, and if you touch them, they lose their magic! (That was a late night for me. Every place was closed, and ours wasn't an official Elf on a Shelf so it was pretty much impossible to replace. I ended up fashioning new clothes out of my fabric scrap bag that night to cover up the burnt parts, so he'd be ready to go by morning.)