We have a Britax evolva 123 for use in our other car but thought we should get an isofix car seat for the main car for added safety. This head support for our priorifix is quite soft and of doubtful additional support for our baby's head. The Britax which is much cheaper provides more support without having to buy anything additional. Also, it did not mention on the product page for this head support that it can only be used on the lowest shoulder strap setting in the priorifix- this drastically reduces its length of use. He has only been in the car seat for 1 week, but we may have to raise the shoulder strap setting and hence, remove the head support.
Maybe a waste of money??