Hi all SGS users,
Some feedback and report after apply ZAGG full body cover invisible shield.
1) Good in protect. Especially for the back cover and side. (Can go for naked without silicon case, cover...bla bla)
2) Reduce finger print a lot. Especially for the back cover

(but when and apply, some dust stick to the shield

)
3) Better grip feel for you SGS. Wouldn't feel slippy anymore.
4) Easy to install.
5) Ok, for the screen side. Actually Zagg shield is really a good protect for the screen.
a) Clarity of the screen --> I will give Good. Clear and high clarity. (Not sure compare to MF)
b) Sensitivity --> Actually Zagg shield does not alter the function or sensitivity of touch screens. Really. (Better explanation at below)
C) Stylus for swype writting --> Same as above, still can work perfectly.
BUT.. BUT... because of the original material of the Zagg shield and make the surface feel a bit GRIPPY. So it reduce the smoothness of the screen and this make you feel that your screen sensitivity reduce and the smoothness to use swype.
So my conclusion, Zagg shield is really good in protection and reduce finger print. For the back cover.
For then screen protect is good as well, but frankly speaking I really don't like the grippy feel on the screen that reduce the smoothness.
I think I will go for MF for the screen protect and Zagg for the back protect. (My wallet burn a big hole

)
NOTE : I USE "MULTITOUCH VISUAL TEST 1.1" FOR THE SENSITIVITY AND SMOOTHNESS TEST, BEFORE AND AFTER TEST. (Can download at market)
Hope this feedback can help you guys for make the decision to buy which SP!

I think the best combination is Martin Field for the screen protect and Zagg Full Body Maximum Coverage for the back protect. Not casing needed.