First, if you never done it before, have a professional do it. If your car is black, DEFINITELY have someone else do it (IMO). Any slight bubble will be seen VERY clearly, whereas with silver, it is much harder to see.
I did my silver one myself. Did the complete front fascia, 2' back on the hood, 2' back on the front fenders, the whole sidesills including where your heels click them, the inside fender lips, and the back bumper where rocks can fly up and hit. Basically covered everything that can be hit by rocks from the tires or someone in front.
Cost me $300 for material. I would assume it would cost that much in labor to have it installed, maybe a little more. Took me 1.5 days to do it myself, but I am slow.
I think it is a VERY good idea. It will be very hard to get a rock chip now.