I think they are a brilliant invention.
It can take a bit of experimenting to get you bait mix right but when you do it works a treat.
I have had success by scalding some halibut pellets then mixing in groundbait like swim stim or something similar.
Get the mix so that it holds together without being too sticky (otherwise it'll stick to the inside). Compress it as hard as you can and then leave them to dry out or I have microwaved them when in a hurry.