Thanks for the kind comments, guys!
Fozzie; First impressions is that the lens is very nice. It seems to be very sharp to infinity. Focus is very fast for a macro lens. The focal length makes it much nicer, more comfortable to shoot bugs and other stuff that is a pain to get within the very close proximity required by other macro lenses. About the only thing that might be better is the 80-400 with the 500d or extension tubes, because of the ability to zoom for composition. But, using the 80-400 that way isn't quite as nice, because you're very limited as to the working distance.
Petal666; My experience with using flash for macro is rather limited, but thus far, I see no significant difference with respect to ambient light with a 1 stop difference in shutter speed. Camera shake with a macro lens at 1:1 magnification is not a trivial issue at these focal lengths. It's not the same magnification as a 180mm at infinity. Only very bright, direct sunlight, such as the snail shot, provides sufficient ambient light to reduce the amount of flash to even out the exposure in the frame. Most macro subjects aren't as willing to sit out in direct sunlight for the shot.
The flower shot above, was shot in deep shade.
In the end, the problem is simply that technology doesn't yet exist to properly expose most of these macro subjects in ambient light, at f/32, on a hand held or even monopod shot. If we had extremely high ISO that we could use, like ISO 32000, then it would be different.
There are other issues as well. Eliminating motion blur on the wings of flying insects requires high shutter speeds. Lastly, I like the vibrant colors and subject isolation that comes with using the flash. Until I can figure out a better way, I'm more than happy with the associated tradeoffs.