I tried the Zagg on my wife's. Was not impressed. Installation was 'eyeball', I don't believe it was glass by the way it lay down. Got the one from Belkin. Has a wonderful fixture for installation, came on clean, no bubbles, glass....impressed enough I got one for myself though I was initially against any protectors on the watch. I have Zagg glass on all on our other devices. They missed the mark on the S2 line as I believe they do have a glass with black frame that they specify only fits S1. Not sure why as I thought dimensions were pretty much identical.
I used the one from Armorsuit. It goes on easily when following the directions. The box comes with 2 screen protectors. Fortunately for me, after installing the first one I slammed my watch face on the doorjam and marked the screen protector instead of my watch. The downside is that within a few months the corner/edge started to lift which I think was due to my shirt sleeves or coat sleeves rubbing on the watch. I applied the second screen protector and a few months later it also started to lift. I now decided it's not worth having the protector and I'm more careful with how/where I move my watch arm.