I was looking for a cheap 70-300 zoom lens for my first DSLR, a Pentax K100D. I decided to try the Tamron as i found it in a sale for silly money. I had read reviews that the lens suffered from purple fringing. This is when there is a purple outline on edges where there is a big differenece in contrast. For example you would find it on leaves shot against a bright sky on some lenses. I was not too bothered for the price i paid as i could always correct in software. To my surprise the lens i had did not show any PF worth mentioning! To top that it had very sharp focus right up to 300mm and the contrast was ok. It was easy to boost contrast a bit with software on the PC. The macro focus is half life size and did a pretty good job. I added a very cheap Sigma Achromatic macro filter which allowed me to focus twice as close so in fact getting 1:1 macro ratio. I also added a Tamron SP 1.4x tele converter and the images were far better than they should have been though focus was a bit slower using the TC. I posted many samples on web forums and only a hand full of people who own this lens could get great results like i did and this must come down to sample variation of the lens as well as quality control. The macro mode is a bit fidgety to get out of but you soon get used to it. If you decide to try the lens shoot a scene that shows any PF so you can see if you have a good or bad copy of the lens. Comes with a lens hood which helps with contrast. Together with the shake reduction built into the camera this combo proved to be far batter than it should! Sold it when i sold the Pentax system and new owner is more than happy with it.
First off this is a budget tele zoom lens. The market really only has two, this and sigma's version.
Overall this is tied with Sigma for image quality, or so reviews indicate. Supposedly this is sharper but has more chromatic aberration at 300mm, while the Sigma has less CA but also a bit less sharpness.
I have tried this lens, but not Sigma's version. This lens does indeed have CA, quite noticeable for certain subjects (high contrast borders). Sharpness is indeed very good, especially considering that this range of the market was usually not great at all.
Build quality is good, and you will be well impressed by the heft of the lens, which is bigger than the Sigma.
If you have Pentax kit lens, its about the same build quality, which is good.
The "macro" feature is useful, especially for bugs and the like, although of course its not real macro. Sigma also has this.
Focusing is a bit slow on my K10D, it should be better on more recent Pentax models, since ,motors are said to be better (K20D, K-7, K-5). Take this into account if you are considering this for sports.
Also concerning sports, particularly indoors or other uses for which you would like to have a faster lens (i.e. lower F number), this lens is not the option. It is slow (particularly so at the longer range), which is natural for this price range.
I must point out that your options for a tele-zoom on the cheap also include Pentax's 55-300mm which I now have. Its a bit more money, does not do 1:2 macro, but I's say its the better choice. Its sharper by a small lead, but much better in CA control, has quick-shift (the DA, not DA-L version) and is also smaller. It also of course has better zoom range.