My first macro lens was the Tamron 90mm and it is a decent lens. I have added several Nikkor lenses to my collection over the past couple of years, including the Nikkor f/2.8 105mm micro and 40mm f/2.8 micro lenses. The 105mm lens is the best of the three and I use it the most. The 105mm is heavier than the Tamron, but is by far the better lens. I also LOVE the 40mm lens as it is light weight, VERY affordable and super sharp - I use this lens a lot for up-close macro shots.
hth
-Terry