(Another year and one repair later, I still love this lens. Original review first followed by updates.)
First impression: right out of the box, this lens is smaller than I pictured. I had experience with the Sigma 105mm, and this is easily smaller and lighter. For travel purposes, it is ideal, plastic build or not.
I did get a few surprises when I put it on the camera (D70), so anyone thinking about purchasing this lens should pay attention. This info is already out there, I just didn't notice and was almost disappointed at first.
1: This lens is only 2.8 at 10 feet to infinity. The maximum aperture drops as you focus closer, and at minimum focusing distance, where many will want to work to get the true 1:1, it is all the way down to 5.6. That was quite a shock at first, but I soon found that at such close range, even 5.6 gives you very shallow depth of field. It just means less speed, so a tripod will almost always be necessary unless you are really good at hand-holding.
2: The switch between auto and manual focus requires you to push or pull the focus ring. I should have known this after working with the Sigma 105mm, but it is not obvious on this lens and I had to check the instructions. lol.
Again, this information is in other reviews, and it may be obvious to most users, but anyone new to macro should keep it in mind to avoid any surprises.
UPDATE: I've left off the old review taken from my first few shots, as I have much more experience to pull from now.
This lens captures beautiful photos, that much is certain. I have not tested the Nikon 105mm VR lens, but from the examples I've seen, this lens can still hold its own, even without VR. (I used to think VR was meaningless on a macro lens, but I have found many times when it would be useful.)
* Bokeh (out of focus rendering) is stunning
* Color is rich (super color with D70, can't wait to see what it does with a D300!)
* Lens is light, less than half the weight of the Nikon 105mm VR
* Focus ring is large and easy to use
* Auto-focus is slow, noisy, and hunts a bit at times
* Lens is not fixed length, short when focusing at infinity, extends out when focusing closer, a significant amount
* Plastic build (doesn't bother me, but some don't like it)
For my money, the pros far outweigh the cons here. I use this lens a lot for product photography (at work), non-macro flower and other small items, but I almost never take it all the way to 1:1 because I'm often too lazy to get out the tripod, or not in a situation where a tripod is usable.
One thing to note: auto-focus has recently started acting up on my D70. For a few weeks it would refuse to auto-focus, I would reboot the camera, take the lens off and put it on again, and it would start working until I turned the camera off once more. Most recently I have not been able to get the auto-focus to work at all. (It is the only lens in my kit that does this, so I don't believe it is the camera.) Since the auto-focus is always slow anyway, I've taken to keeping it in manual focus and just dealing with it. It's a hassle at times, but it does not detract from my enjoyment of this lens.
UPDATE #2: Tamron warranty and service are incredible!
As mentioned above, the auto-focus eventually stopped working completely with both my D70 and D300. All Tamron USA lenses come with a 6 year warranty (6 years!) so I printed a receipt off Amazon, filled out a simple form, and put it in the mail. A couple weeks later I received notice that it was being repaired, no charge of course, and it gave me an order number to track it.
I was leaving on a trip a few days later and didn't want it sitting outside my house while I was gone, so I contacted Tamron service department to see if I could change the shipping address. It had already shipped out and would be at my house within the week. Unlucky for me it arrived the day I left, but I had someone take it inside for me the next day, so no worries. It has been tested and works perfectly! 5 additional stars for Tamron's repair department!
I would recommend this lens to anyone wanting to do macro work, portraiture, product photography, or any not-quite-macro nature shots with lots of color.