I've used the Merit version before, but the rubber sucker dried out a bit, so now it's a PITA to get it to stick. I resorted to taping it on. Generally worked well, though. I've been thinking of the clip-on Gehmann version as a replacement.
Without an aperture, I'm able to focus on the front sight, and the target's acceptably fuzzy. The aperture helps clear the target up a bit, but I don't think that was the main advantage for me, since the target really doesn't have to be clear. I started noticing that with standard shooting glasses, I'm no more accurate with longer sight radii, and I suspect that the aperture was helping me align the sights better by clearing up the rear sight a bit.