I have experience on both sides of the fence here. I've been a very big Invisible Shield fan for over a year - the first two I bought were purchased in early 2007. I've only purchased iPod related Invisible Shields, but I do buy one for every iPod my wife and I own - I'm up to about 7 purchases now.
I've also had some friends see my iPod with Invisible Shield and end up buying their own, and folks who buy my used iPods on Craigslist are impressed with the protector.
My biggest story is with an iPod nano that "went missing". I looked everywhere for this iPod but could not find it. Eventually I was cleaning up in the computer room and found it behind the computer desk. It had fallen off the back of the desk and sat. For quite some time. There were two dents in the back casing of the iPod - but still no scratches thanks to the Invisible Shield.
I keep my iPods in my pocket without fear.
Now for the bad. While Zagg is usually prompt with orders, I have had bad experiences before. Similar to the OP, I had ordered an Invisible Shield and it didn't come in the time expected. So I emailed Zagg and went back and forth a few times with their Customer Service folks. Finally they gave me a phone number to call and a name to talk to. So I called the guy and he took my name and address and agreed to send me an Invisible Shield. However, same story. Didn't get it. So I called back, and it turns out they -hadn't- sent it for some reason. I gave them a third try, and finally a few days later I had my Invisible Shield.
So they definitely do have occasional shipping issues. I'm not sure if this is purely incompetence based or based on the growth of their company - inexcusable, regardless.
If they get that act together, they'll be okay.
Another word of warning, however. Occasionally you'll get an Invisible Shield that has lines in it - this happened with the last one I bought from WalMart Online. Zagg did step up and send me another one, but told me that this would be the only time they would be able to replace it for free.
So its' a mixed bag. Their customer service sometimes could be better, and the product itself really is difficult to install - I *FULLY* agree with Snakehn in that if you are not a patient person, DO NOT DO IT. But I still stand by the fact that the product is high quality. I couldn't imagine having my iPod without one - I don't miss having a scratchy iPod at all.