I fell for this dealer scam back in 2002 when I bought a new Leon Cupra. I think I paid £250 for a Diamondbrite treatment. After a couple of weeks it was as good as gone and lots of swirl marks were showing. It appeared as if the dealers had quickly washed the car, applied a glaze to fill in the swirls they had inflicted, then smeared the sealant over the top.
A couple of years ago I thought I'd give AG Lifeshine a go on my aging Mk6. After 2 full weekends of meticulous machine polishing, the product was applied exactly as AG directed. All the car has had since then is careful 2BM washing every 5-6 weeks, and I have to say I'm very impressed. Nothing sticks to it that won't come off with a couple of passes of the wash mitt, and the only maring is from where it may have got a little too close to the hedge on narrow lanes. I'm sure was on top would increase the depth of shine, but at least the properly prepared paintwork underneath is still in excellent condition.