The supaguards/ diamondbrites are perhaps worth the £5-£10 you pay on ebay, but i would never pay a dealer to apply it, in fact i don't let them wash it,and i don't let them prep it before i take delivery either though, they rarely apply these kits how they should be applied, and they are known to inflict there own swirls and marring whilst applying such products.
Also if you think any protectant will last more than 6 months then you have sadly been taken in by the hype, most of these things need the use of a rejuvenator of some type to keep them going, which begs the question why not just add your own protection at a massive cost saving of these dealer supplied kits.
Im not trying to knock anyone who has bought it, but in my expereince its just a revenue builder for the dealers