For Zagg IS , Proporta, and Martin Overlay Screen Protector, there are very good and unbiased reviews by 360nextlevel on youtube.

Here are some:
Zagg IS:
1. http://www.youtube.com/watch?v=GigcZpGPsHQ
2. http://www.youtube.com/watch?v=ONj3t8meY2s
3. http://www.youtube.com/watch?v=CvD_wyweWFE

Proporta:
1. http://www.youtube.com/watch?v=xatjnXQSFJg
2. http://www.youtube.com/watch?v=DvqA1G2C-7w

Martin Overlay:
1.http://www.youtube.com/watch?v=K7EBz_v04Xg
2. http://www.youtube.com/watch?v=rk_-InqyrN0