I use my Canon MkIII 2x extender with my 70-200 f2.8L II and I'm really pleased with the results. There's a bit of IQ drop off but not a great amount TBH and the AF speed is also slowed a little but it's still good enough for most subjects. Here's a shot from my 70D, 70-200 and 2x extender taken at 1/400sec, f8 and ISO100 at 400mm. It's a jpeg SOOC with no other PP apart from a crop and I think there's plenty of detail in there. I usually use my Sigma 150-500 OS on my 5D3 and the 70-200 and 2x on my 70D, but on the times I've used the 70-200 and 2x on my 5D3 the results are outstanding.
I'm so happy with the results from my 2x that I've just ordered a 1.4x MkIII which I'm looking forward to using with my 70-200 and my 300 f4L IS. If it's as good as this I'll be happy with it.

Yorkshire Bird by
modchild, on Flickr