Let me start by saying I didn't want to like this lens. I've been a Canon lens purist since my first camera. I've always considered second party lenses to be inferior and not worth my time. Recently, however, I acquired another body (EOS 1). I wanted a dedicated walk around lens for it, since I'd been alternating my 24-70L between my digital and this new body. Unfortunately, my funds were a bit limited at the time so I wasn't able to purchase another 24-70L as I would have liked. So I began an extensive search for a lens that would fit my needs AND budget.
After a while, I started leaning toward the Canon 28-105 f/3.5-4.5. However, although I got close to purchasing one several times, I just couldn't pull the trigger. It seemed good enough, but I just wasn't convinced. Then just by accident, I read a review of the Tamron. The review went on and on about the performance and sharpness of the lens. I thought it was all just hot air. Then I saw a second review that stated the same thing. I became intrigued. I started my research. I went to several different sources and a clear picture began to emerge: this is a lens that a lot of people really like. This is a lens that consistently gets compared to my beloved 24-70L, and consistently holds its own--or even out performs it. At first I thought it unfair to compare it to the 24-70L, but then I saw sample images. I saw sharpness tests. I was impressed.
So I took the plunge, and I'm so glad I did. As soon as it arrived, the first thing I noticed was that it did not feel like a cheap lens. I had expected it to be feather-light and almost flimsy. Instead, it feel substantial. It also came with a lens hood.
I threw it on my 400D and fired off some shots and took a look. I was very happy with the results. The color reproduction was great. And so was the sharpness. Just for my own edification, I set up my tripod and made some test shots with the Tamron and then the same shots with the 24-70L. The first go 'round was done at f/5.6. I made exposures at 28mm, 35mm, 50mm, and 70mm with each camera. I had a friend rename the files so I wouldn't know which shots came from which camera, so I couldn't claim bias either way. I then examined the shots. It didn't take nit-picky scrutiny to see the differences. After I selected the photos I found to be sharper in each group, my friend gave me the exif data on each one.
Here is what I found:
First Impression--At 25% magnification on my screen, all the photos looked fantastic. This puzzled me because I figured I'd be able to spot the 24-70L right away. I really couldn't tell the difference.
Then I zoomed in to 100% and the fun really began.
28mm--The difference in sharpness was very pronounced at this focal length. One photo was the clear winner, and that was the Tamron, particularly in center sharpness.
35mm--Again, one of the photos was a clear winner, and again it was the Tamron.
50mm--For the third time, the Tamron's sharpness outperformed the Canon. I was beginning to really develop an affinity for this little baby.
70mm--At this length, the photos were pretty comparable. I picked one, however, and, to my surprise, it was the Tamron once again.
After these tests I felt like a heretic. I had preferred the Tamron lens over the Canon in each trial. Granted, there are more I need to do, from wide open to fully stopped down. But this initial test has made me very happy.
I have seen no evidence of CA so far, and given Tamron's claims, I would have been surprised if I had.
The AF is a touch on the loud side (no USM), but it works well and is accurate. A bit of noise from the AF is nothing to complain about when one sees the results in the photos.
It's not weather sealed like the 24-70L, but that is not as important to me as to someone who is out in extreme weather with some regularity.
Finally, given the fact that this lens is just a third of the price of the 24-70L, and given that I am so impressed with the results, I'd have to say this is one of the better purchases I've made in a long, long time.
I know all Tamron lenses do not perform this admirably, just like all Canons are not "L" class. However, I will certainly add them to my research when I have a new need for a lens.