I recently sold my 300/4 L IS and bought a second hand 400/4 DO, because 300mm was sometime short and I have now also the new 70-300/4-5.6 L. I find the 400DO very useful, sharp enough, very fast and portable. I was skeptic before buying this lens, some people are saying it has poor contrast, but some others are believe it is a great lens. In my opinion the main problem is the list price, around 5000$ or â‚¬ here in Europe. But used is affordable and I am happy of my purchase. Especially now that Canon is increasing the prices of all their new tele-lens, better look at what is available now!