Comments about Canon Canon EF 400mm f/5.6L USM AutoFocus Telephoto Lens with Built-in Hood & Case - Gray Market:

I struggled long and hard about what super-lens to buy. I looked at all of the zooms from Canon, Sigma and Tamron. I tried the Tamron 200-500 and the Canon 100-400L. I liked both, but I have found with this Canon 40D I am much more picky with sharpness than I have been in the past. So this time I went for the sharpness, and gave up the versatility of a zoom. I already have the 70-300 Canon on the long end, so I went for the ultimate in a relatively affordable long lens.
The sharpness is what everyone says it is. Better sharpness and contrast than any other lens I own. I took some auto racing pictures in less than ideal lighting conditions, but the keepers were terrific.

I bought a used one from Adorama which was almost like new and saved about $200. No complaints here.

Comments about Canon Canon EF 400mm f/5.6L USM AutoFocus Telephoto Lens with Built-in Hood & Case - Gray Market:

Picture quality, sharpness, bookeh, contrast is great. Very little flare can be noticed if shoot wide open against strong light, like a sunset. Vignetting can be seen when shoot wide open, I would say this vignetting is an average, but a 70-200 2.8 has less.
Lens is relatively lightweight. Built in hood is great idea although it got very sticky in high humidity like around the eqautor. Focus speed on 5D is average, but faster on my EOS 1V film body.
Lens is compatible with very good results with EF 1.4X. You do need a tripod or monopod when using it with 1.4X, and keep shutter speeds above 1/250.
When shooting handheld 1/400 speeds or faster needed. This is only achievable with higher ISO 640, 800, 1600, or in strong daylight. Shooting wildlife with it in early morning is most likely doomed. There must be a marketing decision that Canon have not updated this with IS. It would probably be able to compete (assuming its price compared to its quality) with the prime super telephoto lenses.
It also doesn't have dust proof rubber or weatherproof housing...which is very necessary in condition that this lens may be used.
It is great to carry while you traveling since it is compact enough and fits easily in my Lowepro Vertex 300 either horizontaly either in vertical position.
Unfortunately as much as I like it as much I hate it also. It will probably sold and replaced with a 300mm 2.8 if time and money comes.

Comments about Canon Canon EF 400mm f/5.6L USM AutoFocus Telephoto Lens with Built-in Hood & Case - Gray Market:

This lens is very sharp, particularly at f5.6 and slightly less at f8. It is very quick handling, making it an excellent choice for most wildlife, particularly birds as a hand held lens for most daylight shooting.

Disadvantages are that it really would be nice if it focused closer. 13.5 ft is pretty far off for a minimum focusing distance. While a 12 mm extension tube can be used to bring this range down a few feet, it would be much nicer if this lens could focus down to at least 6-7 feet.

My only real complaint on this lens is the rather severe problems I had finding any skylight 1a or UV filters that would not cause this lens to go deporably soft. I had to go through 19 filters of 5 brands to find just one that would not soften the image. Considering the fact that none of them softened the resolution of my Canon 135 mm f2 when tested on that lens, I find this performance disappointing. Since this filter problem cost me significant loss of work and time, I really have to warn a purchaser to be very careful to throughly test any filter placed in front of this lens. (To say I was very upset about the filter issues would be an understatement. There is no legitimate excuse for such a poor performance with a clear optical glass filter, much less 18 different ones that work perfectly on variety of other lenses.)

Since the best performing filter was rather inexpensive and no two filters of the same make and type even performed the same on this 400 mm, I can have no recommendation of a particular filter.

The built in hood will probably reduce the annoyance that the possible filter issue could cause. The hood is very fast and convenient to use. It will provide a reasonable amount of protection against dust and water in many situations. I still prefer to have a protective filter on the front of a lens. So I had to purchase and test filter after filter to find one that this lens would cooperate with. This is the first and only telephoto lens of any type that I have found to have any issues with filters in the 35 years I have been shooting.

I do not know if this problem is restricted to just my particular lens, or if this is an endemic flaw of all the Canon 400 mm f5.6 optics. I just think anyone should know this could be an issue.

Note that without any filter in front of the lens, the performance at f5.6 is truly excellent, coming close to the image quality I get from my 135 f2 and better than any of the 5 Canon 300 mm lenses I have used in recent years.