Along with the new-at-the-time-of-this-writing Canon EOS 1D Mark III and Canon Speedlite 580EX II Flash, Canon has introduced the Canon Off-Camera Shoe Cord OC-E3. As you would expect with any updated product, the Canon Off-Camera Shoe Cord OC-E3 is improved on its predecessor, the Canon Off Shoe Cord 2. How much of an improvement is always the next question as it also carries the typical higher price. My opinion is that it is a very nice improvement - read on for the reasons why.
As can be seen in the above photo, the Canon Off-Camera Shoe Cord OC-E3 sports the same metal foot and lever locking mechanism introduced on the Canon 580EX II Flash. Also noticeable is the hot shoe introduced on the EOS 1D Mark III. Not noticeable in this picture is the new metal threaded insert on the bottom of the still-plastic hot shoe - the old threads were plastic. Aside from the increased strength and reliability of these connections, weather sealing is another huge benefit - though weather-seal-compatible cameras and flashes are required for this feature (again, starting with the 1D Mark III and 580EX II).
The more-ruggedly-built Canon Off-Camera Shoe Cord OC-E3 (above center) weighs 4.2 oz (115g) or .6 oz (15g) more than the Canon Off Shoe Cord 2 (above outside). The 2' coiled wire is the same thickness, but has a more attractive (in my opinion) matte black finish - the difference is somewhat visible above. The size of the foot block is 1.95 x 1.5" (49.7 x 38.1mm) compared to the Shoe Cord 2's 1.95 x 1.27" (49.7 x 32.3mm) (see comparison photo below). The size of the hot shoe block is 1.20 x 1.38" (30 x 35mm) compared to the Shoe Cord 2's 1.1 x 1.19" (28.1 x 30.1mm)
OK. So we've established that the Canon Off-Camera Shoe Cord OC-E3 is better than the Shoe Cord 2, but you may now be asking ... What do you use an Off Camera Shoe Cord for?
One of the easiest ways to improve your flash photography is to get your flash off of the camera's hot shoe. One of the least expensive, most reliable and full-function ways to do this is to use the Canon Off-Camera Shoe Cord OC-E3. Since cords limit flexibility and the 2' extended length of this cord limits flexibility a lot more, off camera show cords are most frequently used with a flash bracket setup such as the following Wimberley Flash Brackets ...
The Canon Off-Camera Shoe Cord OC-E3 mounts on your camera's hot shoe, the OC-E3's hot shoe mounts onto the flash bracket via the threaded insert and the flash mounts in the OC-E3's hot shoe. The flash is generally positioned over the top of the lens to send shadows downward and/or to prevent a large lens from blocking the light from the flash. Brackets are often used for macro photography as well.
The OC-E3 maintains all of the on-camera functions for the flash - shoot like it is on the camera. All EOS cameras and EOS compatible flashes are compatible with the Canon Off-Camera Shoe Cord OC-E3.