There have been a lot of posts asking about the quality of the Tamron 200-400 zoom which just came out, and precious little in the way of hands on reviews. There have been some glowing reviews of the lens in the Photo magazines, but since none of the reviews I've seen have any hard data to support the written opinions (and given the coziness between advertisers and photo magazines) I don't think said reviews have much credibility.
I had a chance to try out the new Tamron lens at a photo store, and after playing with it for about 15 minutes, I bought it.
I should explain that I've been looking to get a longer lens for some time. When I bought my Camera (Canon A2) a year ago, I got the Canon 75-300 f4-5.6 with it. This lens is the basis for my comparison to the Tamron 200-400, and therefore I give my following opinions of it:
The 75-300 is not too expensive, has fast USM focusing, is relatively small and lightweight, and is quite sharp at the 75 end.
Not too sharp at the 300 end, and this is really bad since that's where I was using it most of the time. I have a different lens for portrait work, so the 200 + end was where I was mostly using the lens. By itself at 300, it wasn't too great, With a 1.4X teleconverter it was noticably bad, even to my forgiving eye, even in 4x6 prints, as well as being horribly slow. Also: front element rotates during focusing, no distance scale marked on lens. Also, 300mm is just barely where wildlife photography becomes possible, and I found that I was always wanting something just a little longer.
Before I talk about my opinions of the 200-400 let me point out that there has been a lot of discussion by people saying that only a prime lens can come close to the quality they would want at this focal length, and that f5.6 is terribly slow. I won't disagree that a prime lens would be better optically and probably faster, but one should also take a look at the physical size and the cost of a long prime lens. A Canon or Nikon prime lens 400mm and up will be expensive, large, and heavy. Third party lenses will be less expensive, but still large and heavy. Faster lenses will be much more expensive, and considerably larger and heavier.
Focusing is fast, comparable to the 75-300 USM, and quite quiet, except if the lens hunts, at which point it is noticable, but not objectionable. Keep in mind that while USM lenses are silent to humans, they are audible to most animals, so for wildlife photography the focusing noise the Tamron lens makes is not an issue. The front element does not rotate since the lens uses internal focusing. There is a distance scale marked on the lens. I found the optical quality good. I just shot my first test roll. I had 100 speed fuji print film in my camera, so that's what I shot. I went to a prairie area near my house and photographed birds. It was a bright sunny day, and I shot on a tripod and also some handheld, and a handful with the Tamron 1.4x teleconverter. I had 4x6 prints made, and while this is not as good as projecting slides or making 8x10 prints, I feel it is good enough for a first impression review. All of the prints looked very good to me. Good contrast, good detail, no vignetting, and no distortion. I'm sure that at large sizes problems would become evident, but critical examination of bird pictures at 4x6 reveals no problems. [Follow up note: photographing a bright background wide open at 400 with the teleconverter will cause slight vignetting in the corners]
It is, as people have said, somewhat slow at f5.6, so take this into consideration. It is larger and weighs close to 3 pounds, which is another point to keep in mind.
Someone posted to the net that a european review found the lens was not a fixed f5.6 throughout its zoom range. This is true. While the lens reports being f5.6 at the 200mm zoom range, it is in fact f4.5 (a half stop faster) at 200 and f5.6 at 400mm. If you're using TTL metering, this should not be a problem, and in fact is a good thing. Certainly would be better if the lens gave true aperture, but a half stop difference is not a big deal (to me anyway).
Optical quality with the Tamron 1.4x teleconverter is okay, probably not good enough for publication, but not bad. Much better than with the 75-300 Canon lens. With the lens stopped down and on a tripod with a fast shutter speed the quality could be good enough, depending on what 'good enough' means to you. Part of the problem is that you have an f8 560mm lens, so getting a fast enough shutter speed to avoid vibration blur is a problem. Haven't tried this enough to really know.
The lens is small enough when zoomed in to 200mm to fit in a camera bag and carried on a hike. You can do this, but keep in mind that it does weigh close to 3 pounds and it takes 77mm filters, so I wouldn't call it small and light.
In summary, I like the lens, it fills a nice gap between all of the 70/75/100 - 300 zooms and the much more expensive prime lenses. Optical quality seems fairly good, as does the mechanical quality. Whether this lens is for you depends on how much you are willing to spend, and what optical quality is good enough for you. I would say that if you buy this lens and don't already own a tripod, now you will have to buy a tripod. The focal length and the maximum aperture pretty much require it.
These are my first impressions, your mileage may vary.