I picked up a Sigma TC 2001 2x today and so far I'm still floored by how sharp this lens is even with the TC. AF seemed to be great but I haven't done any tracking with it yet.
Here's some 600mm 5.6 shots..nothing fancy just some test shoots. I tried the TC on the 70-200 MK II and I see hardly any loss in IQ. The sigma is a really nice TC.
view them on flickr for higher quality! Tomorrow Im going to go find some eagles down on the river!