I've used them for years on various devices (the GPS I hike with for example). With the newer gorilla-glass (and such) used on phones, I don't bother with them (never got a scratch on my EVO despite keeping in my pocket with keys, for example).
But if you're prone to scratching your screens, it's probably worth the investment. Plus, they have a lifetime warranty, so if the protector gets banged up, they'll just send you a new one, free of charge. I've used that for several of them since I tend to drop my GPS on rocks and stuff and the shield gets nicked up after a while.