I'd say you're ok as long as you don't plan on being underneath the car with just the jack holding it up. My assumption is that your going to jack it up with the extenders and then put jack stands under the car to hold it up.
I hope everybody uses jackstands.... just like jerkyboy says and these others say... blocks of wood work ok but I feel they are too slippery and slide out especially if the car isn't in neutral one or the other has to give, { jack or car I mean }...... I think they are a good I dea I would try them