J.B.
Are you talking about the thing that turns your bike into a tandem? It's a wheel with handle bars and pedals? We use one and love it. My step-daughter didn't learn how to ride a bike until she was pretty old (11) so we got it for her so that she could join us for family bike rides and it was great. My 7-year-old used it a lot last summer and it helped him make the transition from training wheels to two wheels. We had tried many times before but he just wasn't getting it. After a few weeks of being on the tandem regularly, he was ready to try two wheels again and was successful. My youngest, age 4 last summer, also used the tandem even though his feet didn't reach the pedals. He still had fun on the bike, and even though he can ride on his own his teeny bike can't keep up with us so we still use it on family rides.
We have also used the other kind of trailer where the kids sit and get towed behind (like a little bug) and that was very cool, but more for toddlers.