I am in the market to upgrade to a bigger zoom lens, the two I have narrowed it down to are:
Canon 70-300 f4-5.6 L IS
Canon 100-400 f4.5-5.6 L IS
Both have their advantages and disadvantages. It is speculated the 100-400 will be updated very soon, so I am trying to be as patient as possible. The 70-300 however, is significantly smaller and I am hearing that is performing much better than anticipated. There are two new reviews out on the 70-300 and I am very impressed with the quality of images and feedback from each photographer.
Be sure to check it out if you are in the market for a new Canon Zoom:
Canon Rumors Review of 70-300L
Eric Meola's Review of 70-300L