Haha! Same Quiver story here... I have two laying around.

Ha, well I have only one laying around, and it's soon to be gone, because despite my best efforts, I cannot get it to operate as intended.

To be fair, I think this is a 'timing' or 'pace' thing. If you riffle through them at just the right speed, it works. But it's not easy to hit every time. Too fast, and the spectators miss the cards that were intended for them to see. Too slow and it looks like what it is. I think it can take a lot of practice before being able to hit that proper speed every time. And, as you alluded to, everyone's eyes are different, so what may work for 80% of your spectators may not work for the other 20% ...Whether it's possibly not hitting that 'sweet spot' of speed during performance, or coming across someone with particularly keen eyes, the result is that it's not going to work every time, which makes it pretty much useless to me. And that's a shame, as I really think it's quite clever.
Also, I think it should've included about 12 to 15 additional 'effect' cards in the pack; because if you want to use it to force one or two cards for some purpose, you don't look like you have a full deck at all.
Unfortunately, that's money lost for me. Good and clever concept...but not 100% accurate by nature and not a good value for the cost.

Well, after shelving this for a few months for the reasons mentioned in the above post I had an itch to pick up the deck last night so I took it out and gave it a riffle. I was super surprised with what I saw in the mirror. It looked fantastic! I found that actually I was trying to be too perfect when doing the riffle which was causing me to be to tense. I think not being tense made me hit the cards that needed to be hit. I hit perfectly every time after that realization. The finger you use to riffle, just make sure it is at the top correct corner and try not using much pressure downwards on the corner when doing the riffle.

I'm super excited and can't wait to now perform this!