I put the Zagg InvisibleShield on my SF and my wife's DX. We are both very satisfied with the product. I fully trust it with keeping my screen safe for years, and see NO evidence of it getting bubbles.
My only complaint, and it is a minor one at that, is the tactile feel of the InvisibleShield. My finger doesn't slide on the screen as easily now, and that causes some problems when I'm trying to slide a webpage, app drawer, or any other screen around. The screen now feels less slick, if that makes any sense.
I also got an inexpensive hard case from Amazon. While it doesn't fit as tightly as I would like, thus letting in some debris, I haven't seen any scratches from it, and will probably just get some self-adhesive felt to improve the fit.
In short...
ZAGG InvisibleShield = Great
Cheap rubberized hard case = just OK