I have a 70-200 2.8 Nikkor lens that I am quite happy with. On rare occasions, I want a longer focal length. (I am not generally a nature or wildlife photographer). After looking at the prices of 400mm FX lenses, I quickly decided a longer lens was not in the cards. So I decided I could make my 70-200 2.8 into a 140-400 5.6 with a TC-20E for less than $500. I expected some degradation of the image (as compared with a 400mm lens), but thought the results would probably be acceptable. (Note: I am shooting with a Nikon D800E, which gives me about 36 mpix.
I was initially pleased. At some point, however, I took a series of images with the TC-20E on my 70-200 at a focal length of 400mm. The subject was about 1/2 mile away over open water. The results were not as clear and sharp as I expected. So, I upgraded my technique to make sure the camera and lens remained rock steady during the exposure and experimented to find f/8 as an optimal aperture. Then I had an idea for a further experiment.
I photographed a distant subject, first with the converter for an effective focal length of 400mm, then without the converter at 200mm. In post processing, I cropped and upsampled (using Photoshop's default algorithm) the 200mm image so that it had the same angle of view as the 400mm image. I was surprised to find that the upsampled 200mm image had more sharpness, clarity and contrast than the 400mm image. I suppose that the upsampling algorithm does some sharpening on the way, so I applied the unsharp mask to the 400mm image to obtain similar results, at least in terms of sharpness. But I was not happy to achieve for several hundred dollars something that is merely similar to what I could have done in software.
I repeated the experiment on a D700 (12 mpix). This time, the lack of sharpness was a bit less noticeable, but still present.
Conclusion: Don't bother with the TC-20E if you're using a high megapixel camera. If your camera has less resolution, you might make out better.