As an avid PS user for many years, I still have yet to learn the layer mask technique. I still do my montages the old fashioned way and use the eraser tool to delete the backgrounds. I use the lasso to remove most of the backgrounds (with a feather applied) and then just use a fairly large, very soft eraser brush with low opacity and slowly take away some of the edges. When I used the masking option last I had the problem with the opacity showing through the entire image like you are having. I'm sure there is some simple explanation, and some day I do intend to learn more about this, but for now the eraser really does work. Here's a layout I did months ago using the eraser.
http://www.scrapbook-bytes.com/galle...00&ppuser=7297
PS, If you do want some of the background to show through, as I did on part of my layout, you can always adjust the opacity of that entire layer later. Also, to avoid backgrounds showing through you can click to the background layer and use the eraser brush to delete part of the background.