a friend and I both own a 400 f4 DO IS. this lens is fantastic and worth the cost imo. i honestly don't understand comments about this lens having low contrast, i really don't. i used the 100-400 f5.6 IS and wasn't wowed with results at 400. i then used a 400 f5.6 L for about 2 years which proved to be very high in contrast and sharpness, not to mention it's feeling of being small and light in use, plus a relatively low price tag. in my experience the 400 f4 DO edges out the 400 f5.6 in every way except when it comes to size/discreetness and hood design. there is no way i am seeing lower contrast or sharpness in real world pictures compared to the 400 f5.6 (or 70-200), whether in flat light or high contrast light. I find the 400 DO's bokeh more pleasing as well, pics have more pop, and color is fantastic.
I carry my equipment most of the time and tend to shoot telephotos. i considered a 500 f4 but it is much much bigger and would definately only leave the house for dedicated birding trips or airshows. it is not a lens you just carry around. that also meant paying for a 500 f4 on top of the 400 f5.6, which would remain my carry around lens. with a 400 f4 DO i could sell my f5.6 to cover the cost, gain a stop, and AF quickly at 560mm with the 1.4x TC . a 300 f2.8 was also considered for it's similar price and weight but I already have a 70-200 f2.8 IS + 1.4x for the 300-ish range (also have a 70-200 non-IS and used to own the f4 non-IS). I decided i prefered a 400 + 1.4x over a 300 + 2x for the 560-600mm range for sharpness and AF speed reasons.
the 400 DO filled a need for portability, reach, IS, and max aperture that the others couldn't. fyi, my model is fairly recent (date code UV) and has been used with crop camera as well as the 5d mkII. have not seen any odd bokeh patterns.