No doubt it's overpriced, but I still think it's a great lens and am glad I own one. I would not be surprised if it's simplicity is why it's so good - minimal optical elements probably makes the IQ consistent between copies, makes it not go out of calibration/alignment, and barely any moving parts must help reliability.
But being an owner it's worth mentioning the negatives:
1) The lack of zoom can make things tricky in some circumstances. But for the record I've never found the MFD an issue.
2) The lack of IS is a bit disappointing. For birds in shade, or getting aircraft prop blur.
3) It does not pack up as small, and can be a bit more difficult for bag selection.
1) The built-in hood is just plain awesome.
2) The 8.5m focus limiter means it will cycle a focus hunt so fast that you hardly notice.
3) The balance/ergonomics of the 400 is miles ahead of the 100-400.
4) The good balance means sharp shots at lower shutter speeds than you'd normally expect for 400mm.
Great comments, Phreeky, thanks for these. I'm considering this lens myself, so your comments as an actual user are quite helpful.
Question: Is the 400/5.6 easily hand-holdable?