This lens is a classical piece for any Canon nature photographer. It is much less expensive respect to the 300 f/2.8, 400 f/4 or 500 f/4 lenses. However the actual price (~1300€ in 2013) is not very low in general. But in my opinion optical quality is great and it is worth of the price. The only alternative, in my opinion, would be a 400 mm f/5.6 fixed focal length with IS (actually not available).
The distinctive feature of this lens is the zoom movement obtained by a pump movement. I find this movement quite easy to use, even if I know that many don't like.

The aperture is variable along the focal lengths, from f/4.5 at 100 mm to f/5.6 at 400 mm. From the tests shown below, image quality is very good even at full aperture. If used with a full frame camera, with larger pixels, image quality is much better respect to using it with an APS-C sized camera with small pixels (like Canon EOS 60D). For best results with a full frame camera, stop down at least 2/3 stop at 100 mm and 1/3 stop at 400 mm. With an APS-C camera the main problem is chromatic aberration at full aperture at 100 mm. In this case I advise to close 1 full stop at 100 mm and 1/3 stop at 400 mm (at 400 mm the image quality fortunately is very good also on APS-C).
At the corners the behavior is good but non perfect because of some chromatic aberration and coma that does not disappear completely even by fully closing the aperture.

If you're considering this lens to be used mainly at 400 mm, then you'll be delighted by the overall image quality. At 100 mm it is a bit less good.

I've tested the lens also with a Canon 1.4x II Extender, finding a noticeable worsening of the image quality. I don't advise to add this extender.

Image stabilization is a very useful feature in nature photography, and an unuseful one in astrophotography. The IS in this lens is a first generation one, and it is rated of about 2 stop improvement, much less than the 4 stops rated for latest lenses. However I've found a great behavior. Look at the test of the Moon I've made using exposures of 1/60, 1/125 and 1/250 s, all with the lens hand-held. All 1/250s images are perfect, nearly half of the shots at 1/125 s are good, only 1 every 4 images at 1/60 s are good.

My testing was made on a clear night of April 2013. Thanks to Giampiero Realmuto for borrowing his lens.