It does have lag when quickly looking around, but so does the display. It works really well when detecting that you're looking through it. It's very light and adds no weight to the body. But this is also a con, as it's made from very light plastics and feels cheap because of it. I also worry about it snapping off when leaving it on when put in my bag. It comes in a nice soft leather-type pouch. But i don't want to take it on and off every time i get my camera out, this could wear the contact's on the hot shoe too.
If you have a Canon G3X, you need one of these. It makes taking clear pictures much easier than using the screen, especially in bright sunlight. Just used it for a trip to Egypt and it made a huge difference.
Yes, it does cost a small fortune, but now you can steady your camera at eye level, not a wobbly arms length. 100% coverage too. If you can't afford one, save up for it, you wont regret it.