It is a pretty small device, neatly tucked into and around the ear. It sits next to my glasses just fine, and is light enough not to cause any pressure.
I think Carrie (CMD) had a type that is water resistant, which can be worn around a pool and will survive a splash, but not a dunking. Water is a problem, even sweat can be. I take it off to mow the lawn, I don't really need to hear the mower louder anyway.
It does not have any tendency to fly off with sudden head movement, it is light enough and the wire is wrapped around the ear enough to hold it in place. It also seems to take being dropped on the floor without a problem. The bigger issue is any event that would actually crush it, like landing on the side of your head, or having it fall off and then stepping on it. It did come with a two year service policy that includes full replacement in the event of total loss.
I have been on a few hikes, and slipping around doesn't send it flying, so that seems okay. I would probably not wear it if I was snowshoeing or anything really active like that. I have also noticed that putting up the hood on a sweatshirt, or wearing any sort of hat or helmet that covers the ears, will tend to produce feedback, so I would take it off in those situations as well.
Glad you all like it.
Steve