InvisibleShield is known to turn yellow. I've used InvisibleShield on all my iPhones and iPads, and have just installed one on my MacBook Air. Like the previous poster said, Zagg has lifetime warranty and you could send the old one in for a new one. You just have to register your old one with them and place a replacement order. They'll take your credit card information and send you a new replacement along with a return envelope for the old ones. If they don't receive the old ones, you're charged for the full price of the replacement ones, I believe, so remember to send those in and you'll only be charged for shipping.
InvisibleShield, I found, was the best for me when it came to protecting from scratches. Over this summer, however, I got tired of the stickiness on the front, so I swapped it with a PowerSupport Matte protector and boy, what a difference it made! I bought another PS Matte for my iPad2 and have been loving it. I still have the InvisibleShield on the back on both devices for protection, however.