Quote Originally Posted by Joel Eade View Post
Sounds like you know the main pros and cons....

The 300 2.8 may be the sharpest lens Canon ever made and it does well with both the 1.4 and 2.0 converters. I have version I of this lens and it is great. I use it for sports occasionally such as baseball and soccer. I also use it for bird photography. In fact, I took some humming bird shots with it this morning and posted them on this forum! It really produces amazing images.

I never owned or used the 400 DO but if you need a 400 prime have you ruled out the 400 f/5.6 prime? it's a very fine lens for a lot less money.

I have not ruled out the 400 f/5.6 but it does not have IS... I have rented the 300 a couple of times and the image quality out of this lens is amazing even with the teleconverters so if the new 400 DO performs closely to the 300, the longer focal lenght will be nice for birds. With an EF 2x it's 800 f/8 wich is really nice for small birds.