What I had noticed with protective coverings (it was possibly 3M) especially on white cars is that the edges tend to go yellow and there was a dull effect on the paintwork on darker coloured cars. Metallic colours seemed to lose their sheen.
Happy reading. 
I had Defendall for 5 years and it never yellowed, hazed or anything else. The early ones did...the new ones don't.
The edges were perfect as long as they were well stuck down when first fitted...otherwise you'd get a line of dirt under the very edge.
When polished, it never shines quite like paint did...but the colour with/without was identical.
You can layer on sealants/waxes like paint though...and get the shine close.
The film can be machine polished too, as long as you use a DA and a mild abraisive.
It certainly works...i'm not sure I'd bother fitting it on a GTI Golf, especially on one you may only keep 2-3 years...however, on something more exotic, that you intend to keep a longer period, I'd certainly have it again.
