Funny you say that as I was at my Canon dealer ONLY yesterday and he was tempting me with a 200-400 as well as a near new 800! Good deal too but I managed to refrain - just.
Be strong my friend...
Wrong advice Dylan ... The right advice (for the 200-400) is cave in and GO FOR IT!!
I sometimes miss my 400/2.8 II for low light (as Edward pointed out), but it is very rare. I have been close to buying the 300/2.8 II, but the low light performance of the 1DX, combined with the flexibility of the 200-400, has kept me off so far. Up here in the North we have good light from 2:30 to 23:30, so the situation is a bit different than if I had spent my time further south.
Of all the lenses I have bought over the years, the 200-400 may be top of my list. If you can live with the size and weight, you´ll love its performance.
2x5DIII, 1DX, 8-15/4L, 24-70/2.8L II, 70-200/2.8L II, 70-300/4-5.6L, 200-400/4L 1.4x, Zeiss 15/2.8, 17/4L TS-E, Zeiss 21/2.8, 24/3.5L TS-E II, Sigma 35/1.4 Art, Sigma 50/1.4 Art, Zeiss Otus 55/1.4, 85/1.2L II, Zeiss 135/2, 600/4L II