A tummy tub is not really worth your money or wasting your time. First of all it takes up space and after you use it a few times you will just go back to what is easier to begin with, a regular bath in the sink or in the bath tub.
The foetal position that this product attempts to push the child into is one that they will adopt naturally without the shape of the product. Infants assume the position wherever they are placed. Also not keen on them being held by the neck!
Honestly, I used a sink. By 6 months (or whenever they can sit up) they can use the tub. They are not little for very long, any money you would spend on a special tub for the baby, take and put in the college fund.