The fit of the back of the EF mount onto the camera body was a bit tight / stiff (but not 'grindy' at least), or I would have given it 5 stars. Otherwise the adapter communicates with my EF 50mm f/2.5 macro. I'm happy.
This item was primarily designed for the Canon EOS 50mm f2.5 Macro 1:2 lens. This adapter allows you to amp your lens to a 1:1 or twice the image resolution. Be sure to add it to your lens before than mounting it to the camera so that your electronics are properly compensated for during metering. While you will loose a f-stop or or two using this adapter I have still made many photos that paid for this accessory many times over. Add a ring lite to your rig and watch things really pop! NOTE: Others have since reported that the 1:2 adapter can also compliment other lenses to a 1.7x ratio such as the 100mm Macro... I might add that using Extension Tubes, Bellows, and 1.4x & 2x converters yield additional interesting image capabilities, but be mindful of f-stop depreciation... I find a Right Angle eyepiece with magnification adds icing to the cake as a viewing accessory!