Worth every cent. Aldi had some not long ago, they had the Conni ones.
I have two, one I liberated from hospital when I had DD, so it's the official Kerri and one I bought at a baby shop, it's the one that folds around.
I use them on my boys beds while night toilet training, they sleep on top of it. I only then have to wash the Conni or whatever brand you have and don't have to wash sheets, air mattresses and so on. They wash easily and you can put them in the dryer for the same night if you don't have two to alternatte.
Then when they are night TT I use them under their sheet for just in case and a bit like a mattress protector if you kids are night sweaters.
I know people who use them for nappy free time for their very little ones too.