When I looked into this about a year ago (there's a thread on here somewhere) I remember reading that some ATSB published studies show that they are not particularly safe in the case of an accident. They will revent junior from becoming a missile, but junior will most likely suffer serious injury.
I cannot recall whether this is due to the fac tthat airline seats are lap belt only, so that if you were travelling in a car with shoulder harness you might be better off, but I am not sure how you'd attach the loop to a such an arrangement.
And BTW I did not end up having to buy one as our aircraft was delivered with 16!
Good luck!
CS
Found it.... More info here...
http://www.pprune.org/forums/d-g-gen...extension.html