I have one. It was disappointing. I type very fast and if the keyboard is not really precise, soft and "springy" (oh, I know what I mean but...), speed turns into frustration. You can only type very fast when you have a fine keyboard, or the "notes in the piano" get tangled.
I plan to buy a hard, small and thin keyboard. I think Sarah or Cheyenne or NiaTrue recommended one, I don't remember now who did. I will search again in the threads and try to get one.
As Boris said, you need to place them in a solid surface, not in your knees.

But if you need to use the keyboard occasionally and you want to fold it and put it into a small place in your carry bag, it may do. If you have to type for a long time, I would not recommend it.