The body is the Canon 5D. The lens is a Canon 300mm F/4 L (not the current Image Stabilization model) and the converter is the Canon 2X converter (not the current Version II model). Click on the image below to see a full-size version (1000 pixels tall).
Overall, it works pretty well. This image was taken wide open (f/4 on the lens, effectively F/8 due to the use of the converter). The camera was on a monopod, not a tripod. Shutter speed was 1/500 second.
The file was created as a raw file and converted with Adobe Camera Raw 4.0. No sharpening has been applied in ACR or in Photoshop for this test.
There is noticeable red fringing in the tripod color locking knob in the lower left corner of the image, but this can be helped with the Lens Correction filter in Photoshop (under Filter/Distort/Lens Correction/Chromatic Aberration).