I had a nightmare with this lens. In theory the 300mm f/4.0L and 1.4x converter combination should have been better than a 400mm f/5.6L.
300/4.0 + 420/5.6 + IS vs 400/5.6 No IS
In practice it didn't work like that at all. I managed to get a few decent shots out of the 300 f/4.0L, but the vast majority were completely unusable. I took it to Canon's service facility in Singapore for checking and calibration, but it was still no better.
The first generation IS is clunky and not very effective. My other IS lenses feel as if a giant pair of hands has grabbed the lens and is holding it still. The 300 f/4L IS helps a little, but not very much.
I sold it and bought the 400 f/5.6L. This lens has been stellar with far superior image quality and consistency. I don't miss IS all that much, but an update with the latest IS would be very welcome.
I can't really understand all the written praise in this thread. Most of the sample images have been downsized so much that there is no detail visible, or horribly over-sharpened in PP. The squirrel shot wasn't bad.
Perhaps I had a particularly bad copy, but after my experience I could never recommend this lens. YMMV. This is just about my own personal experience.http://phil.uk.net/photography/canon_300F4LIS.html