I've only had a couple... my 8yo has had a lot of them :)
TOTALLY not scary (if you're not claustrophobic). Loud? Yep. (They give adults earplugs under the ear protection -goggle type with a radio built in so you can listen to music, kids they use mondo ear protection for and then stream a movie for them to watch and they listen through the ear protection).
I was really nervous the first time about not moving. I'm ADHD I move. That's what I do. Twitchy McWiggler. And I was going to have to lay STILL for half an hour??? I mean, you see that in the movies all the time, right? The whole "don't move, don't move". Well, guess what? They totally prop you into position with super soft foamy things and pillows... and then if they need to reposition you... they wheel you out and redo all your pillows and supports.
And it's not just the area they're scanning they prop up. You get props on the REST of you as well. They tell you to get comfy (after putting a wedge under your knees and basic support... and then they totally "tuck you in" with all kinds of the molding foam pillow supports.
I get cold in MRI's (some people get hot, for them they have a fan, just like a tanning bed)... so I wear wooly socks and adidas track pants (no metal in them at all) under the oh-so-lovely gown.
The MRI machine goes in stages. 3 minutes. 2 minutes. 6 minutes. Each time they change the stage, the attendant chats you up, lets you know how long the next one is, lets you get a little wiggle out if you need to, turns the fan on or off, changes the radio station. Honestly, I wished there was LESS talking... this was my half an hour of "free time", I was in the middle of a good day dream here, man, quit interrupting my alone time!!!
Totally not scary. Just loud. And a lot like a tanning bed.