Hi Rich,
While I have no personal experience with Master Shield, these semi-permanent/permanent coatings can be legitimate. In other words, they're not necessarily "snake oil" or over-hyped waxes/sealants.
However, as a coating, the final result will only look as good as the application and (more importantly) the preparation. Slap a one of these coatings on a finish that has flaws (i.e. buffer trails/halograms, scratches, etc.), and you essentially lock those blemishes in (at least for as long as the product lasts). This is why I personally don't recommend dealer applied solutions like this to friends/family and tell them to either keep up the maintenance themselves (using a variety of of good waxes/sealants) or to have a professional, reputable detailer do the work.
Also, dealers tend to sell these solutions with the idea that you will never have to wax your car again (or at least for as long as they say the product lasts). However, these coatings are not impervious to scratches or other flaws from routine washing/maintenance. Even in a properly prepped and applied scenario, it is important to make sure that your technique and regimen are sound. Any scratches you induce will need to be corrected just like the paint it protects. ....correction means abrasion and abrasion means you are removing some (if not all) of the coating you paid good money for. Some dealers have a warranty that covers such blemishes but again, I am always leery of letting any of those Neanderthal hacks you typically find in the back lot touch my vehicles.
On a side note, there are coatings like these that the average Joe can get a hold of. More recently, a product from Optimum called Opti-Coat was recently released to the general public (once only available to professional detailers). This coating differentiates itself from your typical sealant since it is quite literally permanent. ....meaning, the only way to get it off is by abrasion (i.e. compound/polish). On the other hand, its characteristics are interesting since it is chemically resistant and appears to shed contaminants very easily. It does so well at releasing substances that it won't even hold other products you try to top it with after it cures (even subsequent coats of Opti-Coat). Anyhow, I was following this product for over a year and bought some when they made the release. I'll post up my own experiences in a new thread once I get the chance to start playing with it.
