I have a large number of shots taken with the Tamron 90mm below,
http://www.pbase.com/djwixx/2007_photo_as_and_when
and I bought it based on a lot of examples I saw from other people.
If you check the link below you can find other examples,
http://www.pbase.com/cameras/tamron/sp_ ... 0_28_macro
One thing to be very aware of is that macro lenses need a lot of light and for close up work the depth of field is very tight. I now almost always use it with a speedlight otherwise you can't get the shutter speed you need. It's an excellent lens but does take practice and patience. If I had the choice again I might consider the Nikon 105VR instead.
A cheaper option that may be worth considering and would give you a great all round lens is the 50mm F1.8 then you can add the Nikon 3T or Nikon 4T close up filters to get closer for 1/3 price. The 3T and 4T also work extremely well on the 55-200VR as below with the price being nearer 1/2!
http://www.pbase.com/image/82838145
I don't like all round lenses and have replaced mine with prime lenses, but that's personal preference. I have tried the Tamron 28-300 and now they've released it with VC it sounds like a great option with a good focal range. As with any zoom lens with that kind of range (even the Nikon 18-200VR) it will be 'soft', so nowhere near as sharp as you'd prefer. You may also need to be careful with the aperture as they'll be prone to vignetting at the longer end. The Nikons tend to have a sweet spot at F8 but anything beyond that may be less than ideal.
I've heard a lot of hit and miss stories about Sigma so I don't even look at them, but given how many pros use them, that may just be an old reputation that is now not deserved.