I fell for this scam when I bought my Leon Cupra in 2002. I paid £250 for the Diamondbrite treatment.
To be fair, if you're not as fussy as most of us on here about the condition of your car, then it's ok. It makes the car easier to wash and keeps the car looking about 75% as well as it could if you cared for it yourself. However, it will not prevent swirls and maring that you can easily pick up even with carefull hand washing. You only have to look at the majority of dark coloured cars on the road on a sunny day to see what I mean. These marks will need to be "polished" out and any correction work will remove these sealants anyway.
Personally, a decent wax and carefull washing will look after your car far better.