First impression: This looks almost like it was taken through a microscope or something. The petals look plastic, and there's almost TOO much dimensionality here!
Relation to the Challenge: A great use of Complementary Colors. Fits the challenge perfectly.
Composition: As I hinted at in my "First Impression" I don't care for the composition of this shot. I do like the petals in the foreground at the bottom left. They add an element of 'grounding' to the photo. I think with a slightly different angle, or perhaps some out-of-focus background, the shot would feel better. Finally, there's a little bit of yellow at the top right corner of the picture that should either be cropped out or explored further.
Background: Not much to say here, as you didn't use one. I think if you had used a lower angle and included some backdrop (out of focus) it would have given a better feel to the photo. As it is I feel almost like i'm looking through a microscope at the flower.
Photographic Technique: The image seems soft overall, as if there's nothing in focus. That is one of the most troubling aspects to this photo, it lacks overall sharpness. Second, the lighting seems off overall, the reflectivity of the petals just above the center of the flower is distracting. Perhaps this was shot too close to the subject, closer than the minimum focusing distance for your camera/lens?
Post-Processing: No details given, so I can't comment much. I'd suggest some Unsharp Mask to give more definition to the edges of the petals.
Overall Opinion: A good effort, but the technical faults detract too much for it to be a ribbon contender. The subject has a lot of promise, I espically like the texture of the center of the flower, but the lack of sharpness prevents me from exploring it fully.
Finally, I'd like to ask you to consider critiquing the critique. :) Just as you're here to learn about photograpy, I'd like to learn about giving better critiques. If you have any comments, even if it's just "Thanks!" please drop me a PM.
At first glance I thought that the the focus was way too soft, but looking at the image full size, i think that the focus just about fits perfectly. awesome macro! 7
Better lighting could have really made this image "pop" off the screen. As the picture stands the lighting gives it a dull flat look especially to the purples which I envision as really being a deep rich purple hue. Not to mention some of the sheen on the purple petals causes them to appear "plastic".