Wow... this reminds me of things I used to say a couple years ago. I mean, who would buy a lens made in 1993!?
Well... I can tell you all, if you own'd a GOOD copy of this lens, then you would definitely agree with me that it is an epic lens - as is. *** and it's my favorite lens ***.
To be fair... my first copy wasn't razor sharp, but my 2nd copy was *perfect* at f/5.6. No need to stop it down.
I used it for sports and birding. When you pick up a 7D + 400 f/5.6 and it feels like a feather... when it fits in a small backpack... when you have to handhold it for hours or take it on a long walk... it is just an amazing lens.
IS would be lovely... but I really want to make this clear: unless you've played with a good copy of this lens for any amount of time (like a few days at least), don't tell me it needs an upgrade. It doesn't need one.
But if Canon added IS, I'm sure it would be nice to have. They're not stupid, they could have added it a decade ago.
Honestly Canon hasn't needed to refresh this lens. IS is *bonus* feature when you are shooting outdoors and in daylight. For most action and birding usages, you want a really fast shutter speed anyways to stop the action. I have never seen any motion blur with this lens because I never let the time value drop below 1/250.
IS would certainly be nice-to-have when your subject is stationary and far away, helps with framing. But it's so light and I'm a big guy, so I can keep it really really steady for a long time and not get fatigued like using a 500 f/4 or bigger.
Keep in mind, it's f/5.6 to start with. It's not like it was designed for low-light or something. It's meant to be used in the daytime where you have good light. So why is IS a must?
And compared to the 100-400... at 400mm this lens is much sharper. I think my copy is a little sharper than Bryan's, but still, you can see at 400 the 100-400 is just poor. And for my needs (birding mostly) I'd use it out at 400mm all the time anyways.http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=278&Camera=453&Sample=0&FLI=0&API=0&LensComp=113&CameraComp=453&SampleComp=0&FLIComp=7&APIComp=0
Just look at how simple the lens design is... (page 52, bottom right). Simple is beautiful. This is one of the simplest lenses to design, and that translates to less artifacts that need compensation. http://software.canon-europe.com/files/documents/EF_Lens_Work_Book_3_EN.pdf
For those who don't open the PDF... here is what Canon thinks about this lens
"EF 400mm f/5.6L USM:This is a high-performance 400mm super telephoto lens featuring an extremely light and compact design ideal for photographers requiring high mobility and portability. The optical system incorporates one element made of Super UD glass and one element made of standard UD glass, thus effectively correcting colour aberration and delivering extremely sharp, high-contrast imaging performance.
" - EF Lens Work III
Sharp, high-contrast, light? Awesome.
In conclusion... My 300 f2.8/L IS II now takes the role of my 400 f5.6/L. But I have to defend it's honor, it's amazing as it is.