I've just got this lens today and did some tests.
First impression:
Build quality – excellent! very solid construction, like you expect from an L lens.
Weight – exactly the right weight that makes a perfect balance with the camera body.
AF – fast, fast, fast!!
Minimal focus distance: 0.9 m. but with 68 mm of Ex. tubes you can get really macro
capabilities, magnification of 1:1.3, very close to 1:1 from about 25 cm.
Performance – very sharp even at wide open aperture with good micro contrast that gets better when stopping down. Bokeh – smooth and creamy, colors – natural.
All images are FF, untouched with PS and were taken on a tripod with a release cable and MLU.