I have used the Zagg Invisible Shield (later the HD version) on my phones for several years. I liked the feeling of protection they rendered, but I did not like the feel of it, the fact they eventually yellowed and eventually the corners would lift up.
When I got my S4 I found this one: http://www.amazon.com/ArmorSuit-MilitaryShield-Protector-Lifetime-Replacements/dp/B00BMIG4CC
IMHO it is much, much nicer than the Zagg and it's only a fraction of the price! Also has a lifetime warranty. It comes in 2 flavors: the one above is the regular one, but they also have a "case friendly" version that is slightly smaller so that a case won't push against it. This one doesn't feel "greasy" and honestly, you can hardly tell it's there.