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.