Mar 30 2007: Tested here are probably the two most talked-about and used wildlife telezooms for Canon mount. It is the classic showdown between the "Bigma" and the "Dust Trombone" (whether this nickname is warranted or not, I won't debate it here). To say that these two lenses have their fans and following is quite the understatement. A few years ago I owned the Bigma and really enjoyed its perks (e.g., 10x range, very good optics, build, ring HSM/FTM, price-point). In a sense, I'm "a fan" of the lens myself. But for the past 1.5 years I've had the Canon 100-400 IS and have enjoyed the benefits it offers (e.g., L build and refinement, IS, very swift and silent ring USM AF/FTM, very good optics). So now comes a lens comparison I've been wanting to do for a long time. First, here are the specs:

Sigma Bigma: $999 USD*, 69oz, 86mm, f4 - f36, 40-118" MFD, 1:5, 20 elements in 16 groups, Zoom-Twist [*B&H Pricing]
Dust Trombone: $1,410 USD*, 53 oz, 77mm, f4.5 - f40, 71" MFD, 1:5, 17 elements in 14 groups, Push-Pull [*B&H Pricing]

Focal length to aperture break-down:
Bigma EX: 100mm/f5 - 200mm/f5.6 - 300mm/f5.6 - 400mm/f5.6 [50mm/f4 and 500mm/f6.3]
Canon L: 100mm/f4.5 - 200mm/f5 - 300mm/f5.6 - 400mm/f5.6
*In this test, the Bigma "determined" the tested apertures because it is the "slower" of the two lenses. For example, at 100mm, the EX is at f5 wide open while the L is at f4.5; but since ALL THE TESTS WERE DONE AT WIDE OPEN APERTURES, to make the playing field even, I had to stop the L down slightly to even them up. So the 100mm test shots are all at f5, since that is where the Bigma is wide open. I did the same thing at 200mm, shooting at f5.6 for all tests even though the L is capable of f5. [At 300mm and 400mm, both lenses are wide open at f5.6.]

Test Comments:
*I really like the Bigma and will continue to recommend it, even though the L is slightly better in terms of image quality. The overall optical performance, build quality, tremendous focal range, ring HSM motor with full-time manual over-ride, and robust tripod collar make it rate extremely high on the price-performance index.
*The L's ring USM is slightly faster and quieter, even though the EX is no slouch. Both focus quickly and accurately. When the focus limiter is engaged on the L, AF performance is even more impressive. Too bad the Bigma lacks an AF limiter.
*The twist-versus-pump zooming action is truly a personal preference issue. I initially detested the L's pumping format, but since have grown to prefer it for this particular focal range. The Bigma's zoom ring is very tight (loosens with use and over time), and twisting from 50-500 is definitely not as easy as pushing from 100-400. But again, I think this is really quite subjective and a personal call. I do like the Bigma's twist zoom, but I like the push-pull a bit more, if only for the sake of ease.
*I much prefer the tripod collar on the Bigma. It is big, grippy, and robust. I like grabbing it as it has "grooves" for your fingers to fit right in. It makes carrying the fairly heavy lens around a lot easier.
*The primary and obvious perk of the L is its image stabilization. All my tests were done on a tripod using a wired shutter (and often MLU). So the L's image stabilization was disengaged the entire time. But take away the tripod and the L's image stabilization really comes into play. The L's main perk over the EX is, obviously, image stabilization (and the EX's main perk over the L is price and having 50-100mm/400-500mm). I personally am a very big fan of image stabilization, so the L remains in my bag.
*For those of you who use filters, consider that the L has the common 77mm thread while the Bigma requires the 86mm variety. If you have other lenses with 77mm, you can interchange them easily (such as a circular polarizer). So this is an economic consideration in favor of the L (oddly enough).
*The hoods are supplied with both lenses and perform as expected. In terms of build, I like the EX better as it is built more solidly and doesn't feel like it will snap as easily. On the other hand, the L has the velvet fabric on the inside which is supposed to aid in controlling light reflections and flare. How much a difference it really makes, I'm not so sure. Also, both hoods can reverse-mount when storing away.
*Regarding the Bigma's focal range, it's truly amazing that it can go from 50mm to 500mm (more like 470mm) and still retain a high image quality. Sigma did well with this one.
*The Bigma exhibits a warmer color cast while the Canon L is more "faithful" and cool. For some, the "yellowish" tone that the EX produces can be annoying. Of course, if you shoot RAW, it's not as big a deal.
*In terms of using a 1.4xTC, I attached my Kenko Pro (pins taped) to the Canon and I was able to achieve consistent AF on the 20D, even in not so plentiful light. With the Bigma, it was a little harder (even while set at 400mm f5.6) to get consistent AF one-shot confirmation, with a success rate of roughly 70%, depending on the contrast of the subject. For example, shooting one particular contrasty sign, the L with the 1.4x would lock every time, but the Bigma would hunt back and forth, traversing the entire focus range. Oddly enough, switching to the multipoint AF mode helped quite a bit. But when it did lock focus, I actually found the IQ quite impressive (tripod-mounted), considering it's about 700mm f9.
*Overall, both lenses can deliver very sharp images with excellent color. Depending on the subject distance and AF accuracy of the particular body+lens combination, the lenses are close in optical performance. While the L generally has the edge optically "across the board", only pixel-peeping is going to reveal the differences. In my experience, a comparable analogy is the Sigma 70-200 versus the Canon 70-200 (with the latter having only a very slight advantage in both optics and AF).

Here is my analysis on the Sigma (S) and the Canon (C) after having used various copies of these lenses. "10" being the best:
Resolution (center): C10, S9
Resolution (edge): C10, S7
Contrast: C10, S9
CA: S10, C8
Build: TIE
Handling: C10, S9
AF (noise): C10, S8
AF (speed): C10, S8
AF (lowlight): TIE
Bang for Buck: S10, C9

CONCLUSION: The Bigma gets my "BEST VALUE" selection while Dusty gets my "BEST OVERALL" selection. If you plan to use a tripod/monopod frequently or most of the time, the Bigma might be the lens for you. But if you want the best lens in its class, it's hard to bet against the L.