I am definitely glad that I didn't decide to just mail order it, because when the salesman actually produced the lens, I learned that it was not what I thought it was. My first shock was that it's much bigger than I thought it was. I thought it was about the diameter of a normal lens; it is in fact much bigger. I also thought that its zoom mechanism was internal, so that it would still be short even at its maximum zoom. Turns out that it telescopes quite a bit. One of the reasons I wanted the lens was that I thought that I could take telephoto closeups of people without the people realizing that I was doing it, because I would be standing across the room with a short lens, so it would look like I was just taking a shot of the whole room. Unfortunately, with the lens fully extended it's no less intimidating than my inexpensive 300mm lens. I was also less impressed with the performance of the supposedly high-speed USM ring motor than I expected. I think the lens focused a little quicker than my inexpensive lens, but only slightly. It still took a long time to refocus if the focus distance was way off (i.e., if I switched from a close up subject to a far away one). It was perfectly silent, but the lenses I have are pretty quiet. Finally, I took a couple of test shots in the store and the image stabilization didn't seem to be doing what it should for me: a shot taken at 1/30 with the IS lens seemed to have at least as much motion blur as one taken at 1/60 with the regular lens.
Taking all these together, I realized that I was not ready to spend $1200 on the lens right now after all. But since I was in the camera store, I wanted to look at some other stuff as well.
I was fairly impressed with the Tamron 28-300 zoom lens. The big surprise for me was that it would focus with the objective end of the lens about 10" from the subject. This would make it a pretty good macro lens, and was the main thing that convinced me to buy it.
I used the new lens to take pictures while I was at ConClave, and for the con, I never found that I couldn't get enough magnification to take the picture I wanted, and only once did I have a situation where I couldn't get a wide enough angle to get the picture I wanted without backing up. With the 75-300 lens I am used to having all the telephoto I want but being unable to take wide angle shots without changing leneses; I did appreciate not having to switch lenses.
I was displeased to discover when I came home something that is buried in the fine print of the owner's documentation. The focusing mechanism shortens the effective focal length of the lens when it is focused closer than infinity. Fairly significantly so; the field of view at the closest focusing distance is about 6.5 cm with the Tamron lens, compared to about 8.5 cm with the Canon 75-300 lens, even though the Canon's objective is more than 4' away from the subject (compared to 10" for the Tamron). I think this lens is still worth the $400 I paid for it, but it is disappointing to realize that it's not as cool as I thought it was when I bought it.
Now, of course, I need to cut a new foam insert for my camera case in my copious free time.