Very interesting discussion as I have in mind to go for a good macro as well (not just yet but in the near future I hope) I looked at Photozone and here are the tests for the Sigma, Nikkor and Tamron respectively:
1. http://www.photozone.de/Reviews/304-sigma-af-150mm-f28-apo-ex-hsm-macro-dg-d-review--test-report?start=1
2. http://www.photozone.de/Reviews/224-micro-nikkor-af-s-105mm-f28g-if-ed-vr-review--test-report?start=1
3. http://www.photozone.de/Reviews/282-tamron-af-90mm-f28-di-sp-macro-test-report--review?start=1
There are also sample images and verdict for each lens.