Quote Originally Posted by SmartWombat
Doesn't the F4 and the 1.4x put it on the AF aperture limit?

EF 300mm f/4.0L USM IS ?977.00

I suppose compared to the EF 400mm f/2.8L USM IS, EF 500mm f/4.0L USM IS and EF 600mm f/4.0L USM IS then it's cheap
I read that there's a way to keep the camera from reading the smaller aperture. Tape the last three contacts on the right side of the lens before placing the converter, and the camera doesn't know the converter is there. Theoretically exposure won't actually change since it's still metering what it sees, but your exif data will be a little off.
I haven't actually tried it myself but I read it on Fred Miranda's site somewhere.