After reading some positive comments on the net I decided to buy a Nikon DK-21M Magnifying Eyepiece
for my D40. Not that I felt the standard viewfinder wasn't good enough for my needs, but more likely to see how much better it can get...
So I paid a visit to my local camera shop and got one for under USD 30.00, tax included. Attaching it to the camera was quite easy - just remove the rubber eyepiece cup that comes with the camera and slide the DK-21M in its place. The instructions that came with it were clear and easy to follow.
The DK-21M has a magnification of about 1.17, which increases the total magnification of the D40 from about 80% to about 93-94%. The difference is notable as soon as you look through the upgraded viewfinder - so much so that I actually experienced vignetting!! It looks like the opening of the DK-21M is smaller than the one of the actual viewfinder, so the corners and the sides get a little bit darker... I could read the viewfinder info at the bottom pretty well, but I had to look down a little for that. I don't wear glasses though... After mounting the DK-21M you might need to adjust the diopter.
So far I only tried the DK-21M around the house. I plan to give it a more thorough "evaluation" on my next weekend photo outing and will report back here. One thing is sure - because the DK-21M protrudes more from the camera than the original eyepiece cup, there is less chance to touch the LCD with my cheek while taking photos