Are these shocker any good?

[plangston]

Hi, my back end is a little soft and I have seen these on ebay, I wondered if any one has used these and if they are worth the money, they appear to be a lot cheaper than the other ones that I have seen, I am only doing the back, but the front will follow if they are any good.
http://www.ebay.co.uk/itm/221411617553
thank you