I had the C and 'upgraded' to the 400 DO II.
I can tell you, the DO is sweet, but even compared to that 10x more expensive lens, the Sigma C I had was not bad at all. Sigma at f5.6 400mm IQ is about the same as the 400 DO II at f4. Thant's not bad. In fact I was a little surprised on the test - you know you'd like your $7k lens to be 5x better everywhere than the $1k lens. The Sigma needs to be around f8 at 600ms to be 'sharp', but it's not mushy wide open. And you can get to 600 without having to add the tele. DO AF is quite a bit better and it is a stop faster..
BUT the fact that I'm even comparing them should say something about the value of these 150-600 lenses.
I used the C outside in any weather that I was happy to be out in for an hour. i.e. occasional light shower, but not heavy or constant rain.
If you are not used to the big teles, and are going to hand-hold it a lot, I would definitely advise the C unless you know you need something really beefy. I occasionally wish I hadn't sold mine, but it would have mostly collected dust. Sometimes that zoom range would have been nice though. I would consider owning one again, even with having the DO if my eldest got more seriously into X-country.
Worst thing about the sigma - the hood is a bit flimsy. But it's light and probably not expensive to replace. Have folks had problems with them breaking? Haven't been following those threads.