When I carry in my pocket it is just the Invisible Shield, but when transporting it in a case or bag I put the rubber InCase on. The InCase provides more shock protection. Invisible Shield alone is strong but thin and really wouldn't help to protect against a fall onto concrete or suchlike. It protects more against abrasions.
Fitting... yes well it was a bit painful I admit. I am very happy with the result though - no bubbles or imperfections at all. The corner bits are not ideal though - they are stuck firm enough but the design means you can feel the edges on the iPhone's smooth rounded corners.
During application, I took my time and did not panic as I had done a similar application once before. Allow yourself 30-45 minutes to do it and don't try to hurry. Follow the directions. The material can be manhandled quite a lot during application as long as you keep your fingers wet, so mistakes can be corrected to a certain extent. Anyhow, Google it and you'll find a lot about the application process.
One thought though - I would hesitate to use this product with the 3G - only because of the rounded back. That might make it a bit harder to apply well.
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http://www.everythingicafe.com/cases/body-skins/shield-zone-invisible-shield-20070709251/#user_reviews