I just noticed those in the store for the first time a couple of weeks ago. If they’ve turned you into a knitter, then there must be something to them! 
I’m curious about them. They work like a knitting spool, right? Where you just wrap the working yarn around the outside, then hook the existing stitch over the new yarn?
How do you do a hat? Do you actually decrease for the top, using the different size frames? (I can see how this would work, but it seems like it would be a bit tricky to maneuver.) Or do you just knit a cylinder and bunch it in at the top?
It seems like it would be possible to do knit-purl ribbing on such a contraption (if it has the right number of stitches), but not a row of all purling (not easily that is).
I could see them being fun to use for some projects, but I could never use them, because their fixed number of stitches, and gauge limitations, would drive me nuts! LOL! I like using my double pointed needles, where I can increase and decrease on a whim, and easily switch from knit to purl, and cast-on any number of stitches I need, etc.! But I can see how these frames would be less intimidating, and exciting for a new knitter. I’m curious to see some of the things that have been knit on these, and what they are capable of!
Amy