I've come to the Nielsen Media Lab for a number of paid studies and have consistently had good experiences! Leaving with a $75+ gift card in your hand definitely helps.
Once you're on their email list, you'll get emails when there's a new survey available. After taking the survey, you'll get an email if you qualify for a study and which slots are open (often for the next day). You then just email them back with your two preferred slots. It's not the most high tech scheduling system, and sometimes you won't get a slot if you don't respond quick enough, but it generally works, or they'll email you later with slots for another day.
When you arrive for your study, they give you a quick run-down and then put the EEG cap on your head. They use leave-in conditioner, so your hair will not be looking fresh afterward. Don't plan a date until after you can get home and shower. Trust me. Then they lead you to a room where you'll perform a series of tasks and watch videos. After this, you fill out a quick survey on a computer and can comb your hair to try to make it look less like a mess. You stop by the reception desk to sign off that you received payment, and you're good to go! read more