I don't see any reason get this if you already have a 100-400.
It's a terrific value lens, but the difference in focal length is actually surprisingly small between the two*, and all the 150-500 owners I know pull back to ~400mm nearly all the time for sharper results anyway.
My personal experience (tallies with most comparisons I've read as well) is that the sigma at 400mm f/8 is pretty much as sharp as the canon at 400mm f/5.6, and the canon gets sharper when stopped down.
*The optical design of the sigma means that as focus is pulled in from infinity the focal length gets shorter, making it more like 450mm than 500mm. There isn't a big enough difference to get shots with the sigma that you couldn't get with the canon by just cropping a little bit.
If you're looking for a lens to go alongside your 100-400, my choice would be the 400mm f/5.6 prime. I chose the 100-400 instead of this because most of the time it shoots my shooting style better, but I do think the 400mm would make a nice addition to my collection (maybe some day) for those times when I know I'm not going to want shorter focal lengths, IS or closer focussing like when I'm going out to bird hides. The 400 prime has unbeatable image quality and focus speed in it's price range, a lot of people say it's
the lens for birds in flight since it's so quick and lightweight (I'm sure plenty of users on here can attest to that).
That way you'd have the 400 for when you want the best image quality and the 100-400 for when you want the flexibility of the zoom/IS/closer focussing.
Just my 2p worth
