I bought mine on eBay for a fairly reasonable price in great condition.
I can lug this lens around with my toddler on my back in a kelty backpack on day hikes. And I can shoot handheld!
400 mm is a tough focal length to break into. There is a real line in the sand here where money and weight play a role.
I rented the 100-400 but the 5.6 aperture was too limiting in woodsy areas (ok in better light).
I rented the 500 f4 and it was just too big to take on day hikes with my family. If I were single, this would be my lens.
The 400 f5.6 does not have IS. Since light is not always great and I need to shoot hand held, I did not consider it much.
It certainly is a niche lens, overpriced brand new.
Takes a 1.4x converter very well. I have read and am beginning to agree it will take a 2x converter well if you stop it down one stop and use a tripod. I used it hand held at 800mm and was shocked by the quality of the image. I only wish I had the time to whip out my tripod.
I do need to process raw images with some extra contrast - seems like an overrated complaint. With a long lens I have noticed a bit of back focus so sent the lens and 5Dc in for calibration.
If the used price is fair - get one!