Hi Craig66.
Well, after almost year of using I concluded this:
Good for: light hiking where you don't have large backpack (my is 65 L) and lots of other gear, when the paths towards mountain/hill spots are easy. Excellent on daily use. It has two little metal hooks, so (if and) when it pops out from its slot, it remains with one hook in camera. At least, that was in my case.
Not good for: (semi) extreme hiking, because of the knowledge from light hiking/daily use that there are times when it pops out and I didn't want to have risk of losing it. Problem is when you use it on difficult paths, camera is moving alot and you don't have energy to look every time if the eyecup still standing or not.
Also, before week or two, I noticed that some little part of rubber is cracked, but
I repaired it with super glue on almost whole area from the outside and small one from inside, just to be sure. Now it's ok.
I would like if the producer could make this eyecup as it is BUT with hooking method just like on the eyecup that comes with K-5 II or K-3. That would be much firmer and without any chances to semi pop-out. On the other hand, perhaps producer wanted to be just like this because if it could be the way I want it to be, then only the rubber would pop out, while the metal construction, where rubber is attached, would stay on the camera. Just thinking.
Bottom line: I use it 90% of time, and when I go where I think I could have risk of losing it, I put genuine eyecup which came with K-5 II. It takes 10 seconds. I recommend it.