I had this lens for a few months and tested it at different focal lengths. Here is the break down of things I find about this lens:
Pros:
1. It is the cheapest and lightest lens available for nikon lens at 400mm.
2. The build is solid.
3. The lens hood has a little wheel for you to rotate the filter when you have such as a cpl filter on the lens with the hood on.
4. There is a ring attached to the lens so that you can rotate the camera+lens on a tripod easily.
5. Sharpness is very good with any focal length shorter than or equal to 350mm.
Cons:
1. Sharpness at 400mm is soft. The pictures are almost unacceptable if taken by handheld. However, using a tripod will solve part of the problem, and stopping down to at least f8 will somewhat help as well. BTW, you should shoot with a tripod anyway with such a heavy lens. So my suggestion is to stop down to at least f8 and shoot on a tripod if you want to go to 400mm with this lens. (This statement is not totally true, see the P.S. section for more info)
2. Chromatic aberration is quite obvious with this lens up to 400mm. Compare to my Nikon 18-200mm vr lens, this lens produce significantly more chromatic aberration. I found significant longitudinal and lateral chromatic aberration at even f8 at 400mm. The pictures have mostly either "green fringing" or "blue fringing" instead of "purple fringing" which is usually associated with the camera sensor. I kind of look at the internal glasses design of this lens from Tokina's website. It seems that they use achromatic doublet to improve the color fringing, and it is claimed that the glasses are multi-coated. However, this is still my biggest concern.
One thing I have not tried is to shoot subjects in motion with this lens, so I can't say how fast this lens can be.
P.S.:
7/04/09, I will correct my previous statment that shooting handheld with this lens produce unacceptable images, even at the 400mm end. I think I used my 18-200mm VR lens so often that I can take most static or moving subjects at speed around 1/100 sec. (when the maximum focal length is 200mm) with no sharpness problem. The VR property kind of defys the old rule about the fact that you have to shoot at the speed of at least 1/focal length, and blinded me from following the old rules. I have been shooting again handheld with this lens today. After I came home to look at the images carefully, I realize that some of images look sharp and some don't even when they are taken at the same focal length. The only difference is that they are taken at different speeds. I did more testing and found that even at 400mm, you can get reasonably sharp images with static subjects when you set your shutter at 1/640 sec or faster at handheld. So, it was my technique, not the lens that produce poor results, and it reminds me again that THIS IS NOT A VR LENS and you have to apply the old rules on this lens, especially at 400mm. My final conclusion, you can get sharp picture with this lens even at 400mm if you use it properly.