Personally speaking i think it is pretty good stuff, especially the Alloy wheel protector. makes life a lot easier when cleaning fiddly wheels.
Many other disagree though!
I wouldn't pay a dealer £300 quid to apply diamondbrite or whatever it costs. I do however feel that the actual products such as this and Supagard are actually quite good in what they do. The cost is in the dealer labour and a high profit margin for them. If you can find someone willing to polish and apply the treatments for a more reasonable sum of money then its probably a worthwhile excercise.
try and be-friend a groomer where you buy your car from.... they always have loads left over after doing other cars, that's how i got mine.
I can truely say that the alloy wheel wax is brilliant and made cleaning alloys a breeze!