Tamron, like Sigma and Tokina makes lenses that provide less expensive alternatives to Nikon or Canon optics, often taking a back seat in optical design and/or optical quality. But on occasion these companies manage to build a lens that beats the big boys and often still at a price many can handle.
 
The Tamron 90 and 180mm macro lenses are, in this reviewer's opinion, the best macro lenses made if you judge based on optics. They are stunningly sharp, present nice bokeh and are built well enough to deal with most pro applications. Sigma's 150mm macro is also excellent and features it's own internal HSM focus motor, meaning it will focus on D5000, D40 and D60 models. The Tamron lenses won't, but this is often not a factor for macro work. Nikon's new 105mm AF-S VR lens is a nice item, but in test after test with two samples (and seeing many online samples) I'm convinced that the Tamron lenses are sharper.
 
Build Quality
Perhaps more than any factor, this is where you'll see the biggest difference. Nikon's lenses are built tougher and handle a bit better. But staying realistic, there is very little issue with the build quality of the Tamron. The 90mm feels the cheapest and is very light. The 180 is certainly heavier and handles better. Unlike the 90mm the 180 ships with a tripod collar which works well.
 
Auto Focus
The 90mm has a limiter switch and focus falls short of being quick, but it does well enough. The 180mm has no limiter switch and on occasion it will hunt and go through it's entire focus range to find focus. Since this was never intended for sports shooting, I don't see this as a major failing. But if you want your macro to double in that capacity, the Sigma 150's HSM focus motor is better. I use the Tamron 180 for macro and portraits and the AF is fine for that.
 
Compatibility
The 90mm & 180mm have no internal focus motors, so it will only manually focus on a D40, D60 or D5000.
 
Optics
Well, here we go! If the Tamron 90mm & 180mm are not the best macros, it's certainly close enough. I prefer them to the Sigma 150 and it's sharper than Nikon's 105vr or 200 Micro. It's an amazingly sharp lens. When pressed into action for portraits, the lens impresses yet again, allowing close range work. The 180mm is actually sharper than my 3X more expensive Nikon 70-200vr lens! I suppose it should be. It's a far less complicated prime lens and the 70-200 still has better bokeh and low light ability and the wonderful VR, but credit must be given for the Tamron's sharpness, color and contrast. Simply outstanding. Some might say that the Tamron is too sharp for portraits, but that's an old wives tale.
 
Summary
Considering their range of use, these lenses are not cheap to buy, but they are wonderful tools. Having owned the 90 and now the 180, I'd suggest the 90 for DX cameras and the 180 for FX. Those wanting more working range would still prefer the 180mm on a DX camera. Or buy both. These are two stunning primes by any measure.