I've seen various films on various different cars, and the following is generally true on all of them:
Yes, it's noticeable. Not from a distance, but definitely from arms length.
If it gets hit by a stone, it will mark just like paint. Not as easily, and you still have the paint underneath, but there will be a mark you cannot get rid of.
They do yellow with age, but we're talking years rather than months. With a garaged car that only came out for shows I very much doubt this would ever be an issue, as it's the UV that does the damage.
You'll never get as good a finish as you can with just paint. The shine simply isn't there.
In terms of how well it protects the paint, it does an exceptional job. It will still look battered and worn, but once you peel it off then underneath is immaculate.
I've considered it on most of my cars, and I always come to the conclusion that since a front-end respray is equal to or less than the cost of having the film professionally applied, you might as well just budget for a respray once every couple of years instead.