Wow. Komine hit the nail right on the head with this one. This is the best macro lens I've ever tested. Note: This is the Komine version. There were several versions made by other manufacturers. This review does not apply to those 'other' Vivitar 55mm f2.8 macro lenses.
First, it is razor sharp. At f2.8 it is as tack sharp.
It is a true macro lens. It focus smoothly and seemlessly from infinity down to 1:1. The lens barrel does extend significantly. The lens more rhan doubles in length.
Focus throw is short in normal shooting but quite long as you enter macro range. This is ideal.
The aperture clicks at half and full stops.
Color rendition is superb. Contrast is excellent at all apertures. Shadow detail is excellent. Fringing is minimal. I had to shoot in very harsh light at f2.8 while severely overexposing just to get a noticeable fringe. It will fringe in the corners in harsh backlight at f2.8, but if you are shooting f2.8 into the sun you deserve abberations. Bokeh fringing is very well controlled. Even at f2.8 it is barely noticeable on typed text.
Bokeh is very smooth at wide apertures. I have not tested for small aperture bokeh yet.
The lens handles very well on my T2i. Focus is easy to obtain. The lens is reasonably lightweight, no heavier than a good 50mm. I will be adding pics taken w/ a full frame 6D at a later date. Stay tuned for updates.