I had a Zagg Invisibleshield, but it got slightly detached from a corner and began accumulating dirt under the protector, so I had to remove it after about 4-5 months.

Now I have some super-cheapo (something like 6-8 for the price of a Zagg display-only protector) protectors, I think they are made by Celly.

Celly vs Zagg:
+The Celly has a hard and slippery surface, unlike the Zagg. This makes swiping movements much more pleasant.
-The Celly is clearly thicker, and as such easier to install and feels really sturdy. Unfortunately a thick protector on a resistive touch screen does noticeably degrade touch screen responsivity.
-The Celly is completely hopeless when it comes to resisting fingerprints in any way. Even right after wiping it, it looks greasy.
-These stick to the screen REALLY hard. Some unfortunate soul claimed to have ripped the display off the device while attempting to remove the protector (some protector, hah), but I'd say that's more likely the result of someone just grabbing to protector and trying to pull it straight off the display, instead of gently and in the right direction.

Long story short;
I won't buy either of these again.
I'll be looking for a thin (touch responsiveness), yet hard (ease of install and use) protector with minimal surface friction.