I've used a Zagg on three different phones. There's ups and downs.
The big selling point of these things is they're TOUGH. Zagg makes some of the most robust screen protectors on the market and they're not bad with glare either. Anything hits your screen short of something that would shatter the glass, you'll be protected with a Zagg. The material they use is the same material that's used on helicopter rotors in the desert to prevent the sand from chipping away at the metal. Good stuff.
The down side is they're grippy and finicky. It seems to depend on your environment. Things like humidity paired with how dry your skin is can make your finger glide across the screen with no problem, or jump across it like rubber on wood. When they work they work great, but when they don't it makes your touch screen damn near unusable. All of this experience is with a resistive touch screen. It might be better with capacitive due to the lack of pressure needed, I'm not sure.
Also, they collect crap over time. You probably won't notice until you pull it off to sell or return the phone, but these things turn brown. Probably because of the rubbery porous nature of the material absorbing oils from your skin. Not a huge con because you won't notice it until the thing is off the screen, but some people are picky.
I would buy one for a phone that I thought was at risk for getting easily scratched, like in a really sandy environment, or a factory that makes things specifically designed to easily scratch stuff. If you don't have to deal with these things, I would go with something a little more smooth and glass like.