I don't use a screen protector for either of my iPads. I own an iPad 2 and the new iPad 3. I got both in March just within days of each other.
My iPad 2 is my everyday work iPad. I take it with me in the car on my sales route, have my customers sign on the screen with a stylus, use it for interactive presentations (which my customers physically handle the iPad), eat breakfast and lunch while using the iPad - sauce, oil, and coffee splatters on the screen, wiped off with regular paper towels. Since its my everyday wear & tear device, I even let my cats have a go at it. I installed the app Cat Toys, which is basically a mouse that runs around on the screen and emits a squeak every time it's caught (by tapping on it, and in this case, with paws). My cats love that app and eagerly wait for me to cue it up so they can have at it, claws and teeth!
My iPad 3 is my baby, which I handle with utmost care. I wash my hands before handling it, use only the finest microfiber cloth to gently brush off the smallest speck of dust, and use it in vacuum environment settings as much as possible. I put it in an Otterbox Defender to protect it from the harsh, cruel, outside world. The cats aren't even allowed in the same room it's in!
One day, as I was properly cleaning the pad 2 (microfiber cloth w/ a very light hint of moisture), I noticed that the glass had not one scratch. I inspected it closely under a bright lamp, and it looked identical to my iPad 3. It was then I realized that the gorilla glass can really take some abuse. Since then, I appreciated the iPad for what it is. An everyday device that can entertain, inform, help with tasks and productivity, and for some, sharpen your cats' hunting skills. I've taken the iPad 3 out of the defender case (man I couldn't stand that distorted screen!!!) and put it in a folio case. Now I use it like a normal person would. The cats still aren't allowed to play with it though.
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Cliff notes to the above:
iPad's gorilla glass can withstand a bunch of stuff, including cat claws.