I used a glass screen protector for a few days, but found the edge distracting and it came up too easily.
Since I have the sport watch, I use film protectors. Most of the early ones were either too stiff (wouldn't lay on the edges), peeled off too soon, too small, or too large. Recently I bought Skinomi. The most recent ones fit fairly well. Some gap around the edge. But they lay well against the glass. In my experience with previous versions, they last for 4-8 weeks. It all depends on how often I bang up against something. Air travel is to be the killer for me which is unfortunate since I travel ever month or so.
Skinomi allows you to return older films for a refund. They aren't very expensive on Amazon, about 1 USD each, so I haven't bothered yet. I haven't tried the process yet, but since I'm on my 2nd box of 6, maybe I'll try it.