I think they are the best thing since soft butter, O wait they've been around since before soft butter. I put 2 speed kickbacks on most all of my bikes. Think I got about 14 of them. My first, a blue band, was on my Sting Ray I bought with lawn mowing money back in 1964, wish I had that bike.
I rebuilt one and it work for a while then stuck in low gear, I think I put too much grease in it. I now take them to a LBS for rebuilding, if there is a problem with it he will fix it, and I like to support the locals.
They do take a little getting use to, but a quik kick back and your in the gear you want.