I'm glad you asked this, Igor, as I'm interested to hear opinions as well. I have done some research on macro lenses and from what I've seen, the Tamron 180 is spectacular. On one site it was rated a 5 out of 5, with very few other lenses receiving as high a mark. I just started looking into the Nikkor 200.
Vinh (VPT) uses the Tamron and has posted in the Flowers & Macro forum. He mentioned he wanted to get rid of it but not why. Maybe he'll see this thread and contribute.