I've done two, one at IOA and one at US.
The first was while leaving IOA. They brought me into a room close to the exit gates, and wanted to test animations for a new ride. They then showed me part of the Simpsons Ride (from what I remember it started when the kids are on the kiddie ride and ended just before the big water stunt show). This one was unpaid, but was cool to see a work-in-progress Simpsons ride.
The other was at US. I was approached by Twister, and brought into a room in a building across from where the Blues Brothers perform. This one was a paid survey ($3 in Universal Dollars), and it was testing different body washes.
I can't do them anymore because I've been working for a market research company in NYC for the last 2.5 years, but I always try to get my friends to do it if we get approached. Market Research demos/quota are often a mega pain, so I sympathize greatly lol