I had a Zagg on my Incredible and I have the Steinheil Ultra Crystal on my Thunderbolt. Both have their advantages and disadvantages, and neither is perfect. Since you asked solely about clear/smooth screen protectors, I will address my experience with the Steinheil.
This protector has a pretty impressive fanbase, but in my view, it does have some flaws.
Pros:
1. application is very easy;
2. smoothness is great -- it feels like the native screen; and
3. fit is very good
Cons:
1. scratch resistance: the Steinheil scratches very easily. I am vigilant about keeping my phone away from all sharp objects, but I was surprised to find a large scratch on it recently. It occurred when I had my TB in my backpack and the only thing in the pack with it that wasn't made of cotton was a plastic squeeze container of sunblock. The welded plastic edge of the squeeze container apparently rubbed across the screen once and left a substantial scratch in the screen protector. Fortunately, Steinheil typically provides two rather than just one screen protector with your order, so I simply replaced it
2. Translucency: I used my TB for several weeks without a screen protector and I became accustomed to the way the screen looks in all forms of lighting (sunlight, indoor illumination, darkness, etc.). With the Steinheil in place, the screen looks just as good as it does without a screen protector in all of those lighting scenarios EXCEPT bright sunlight. In sunlight, the protector imparts a hazy appearance to the screen not unlike a light fog. That hazy appearance did not occur when I was running it naked.
I haven't seen much posted about this latter issue, so I expect that some others will disagree. However, I have experienced this with both of my Steinheil protectors, and my application of the protectors was according to the manufacturer's instructions (it was ideal), so it does not appear to be some error on my part or some random individual product defect.
