I'm chatting with the wife about this, and I don't know a thing about them. Anyone ever use one and have any insight? I'm gonna start keeping my eyes open for a used one. I found out they're also called Tag-a-long bikes.
We used one for a while with the boys. They are good for getting kids out for longer distances. Make sure you get the right shim collar for the seat post.
I've been using one for a few years. currently my 5yr old rides on the back, he broke his arm 3 weeks ago so riding a bike by himself is not doable. they work great. Mine is some cheapo brand, bought is used for $30.00. Bonus is that is folds in half.