Zaggs are good if you're the kind of person (like my girlfriend) who just chucks their phones into their bags, and it bashes up against things like keys, coins and other things that could and will scratch the hell out of the screen.
They have a jelly like feel to them, they are soft and will 'indent' when you scratch onto them, so unlike regular screen stickers, zagg's won't scratch. But because they are soft and jelly like, you loose lots of clarity to the screen, and they become all pimpled and potmarked from all the knocks that they absorb.
Basically Zaggs for me are a waste of time, I like my phone to look and feel new, and with a Zagg the phone looks and feels like it's covered with a layer of sticky back plastic we used to use to cover school books as kids. Basically they're horrible