DH loved this when we went, the lines were so short he did it twice in a row, then convinced me to do it with him.
This is a virtual roller coaster, you are first ushered into a room where you have an intro video that explains how to create your own roller coaster. The video was very cute and aimed to get kids into math or engineering. My DH is an engineer so he enjoyed it and thought it was neat.
After you watch the video you are taken into another section where you use touch screens and tools to create your very own roller coaster experience, we did the jet one which can do a lot of looping sequences, etc (we are daredevils), there are also bobsleds and I think a normal roller coaster but I didn't look much at those.
Each rider is given a card (each ride vehicle holds two people, you can do it solo), you swipe the cards on the terminal where you create your roller coaster and the roller coaster gets stored in it. Also you can review your roller coaster once you get home using the card as well
After you are done creating your ride you are ushered into your vehicle, it is a mechanical arm that uses hydraulics, etc to simulate movement, it can put you upside down several times if you so desire. You leave all your loose valuables and bags on a locker outside and are given a key to keep with you to retrieve them when you finish.
You get strapped in good and you can see the face of your partner in a corner of your ride screen, it is very funny to see their reactions if you pay attention.
If you are afraid you might not fit due to size there are demo seats outside of the queue area for you to try on and see if they are too snug or too loose or if you will get scared being in close quarters since they close in front of you.
It was a fun thing to do and DH has declared it his favorite ride in Epcot.