Does anybody have experience in using these lenses and am I right in thinking that, even though they're not as good as a telephoto lens which isn't a converter lens, a good converter lens at full magnification should still produce sharp, well-focused images, assuming, of course, no shake of lens or camera, etc? I have recently bought a telephoto converter lens for my Coolpix 8800 and every shot is enough out of focus to make the picture not worth keeping. Using tripod and self-timer - therefore definitely not due to camera shake.

Can anybody help, please?

Usually, using the converter lens with cameras, especially compact cameras, yields much lower image quality. That's one thing you have to admit. By the way, do you use the genuine converter lens from Nikon? If not, that might be a problem.