I bought the EVF for the G1X II about 6 months after I bought the camera. Yes I leave it on when it is sunny and find it very useful as when I travel as I do not always have the choice of the time of day as to when I can take photos.
I have the small bag Lowepro StreamLine 100 in which I put in the camera; the EVF; the bayonet hood; the glued together 58mm filter adapter (with a polariser screwed to it) to another bayonet hood; spare battery and lens pen cleaner. Since I bought the EVF after the bag it is not configured for the EVF to stay on the flashmount while in the bag.
Once I put on the EVF it normally stays on for lengthy period of time and that is not a problem for me as I use a home made shoulder strap to emulate a BlackRapid.
So far, I did not buy one for my 1G MK II. Most of my photos are indoors where the LCD works ok. When out in sunny weather, its near impossible to accurately frame a image, so its bound to make a big difference.
I may yet buy one, but I mostly use my DSLR in sunny weather outdoors.