The EF 100-400mm f/4.5-5.6L IS is one of my favorite lens in Canon's lineup and my pick for the best value in a wildlife zoom lens. You can read my review here.
Although the present lens is a top performer for me, it has been around since 1998 and can take advantage of some of the latest lens improvement technologies like new coatings and materials. In the past two years, Egami, the Japanese photography blog has turned up at least two new Canon patents pointing to a new EF 100-400mm lens but now it seems Canon may be ready to finally announce the replacement to this venerable zoom.
In light of Nikon's recently released 80-400mm f/4.5-5.6 AF-S zoom lens, Canon is expected to release their EF 100-400 f/4-5.6L IS II lens about the same time as they announce the EOS-7D Mk II camera, perhaps some time in late Summer to early Autumn. The new lens will be heavier and more expensive, but less than 4 pounds and $3,000 respectively. I cannot wait for this combo to come out and will place my pre-order as soon as possible. Where is the EF 200-400mm f/4L 1.4X zoom? I have waited long enough, Canon. Hurry!