I am starting to consider this lens, haven't decided on the new one with OS or an older version yet. My 100-400 has always bothered me being as slow as it is, I think the small aperture affects the AF performance of the 100-400. I am wondering what the Siggy will do for my AF as well as offering the option of a 1.4x TC and still being faster than a Canon 400 f/5.6L. Right now this lens is in the lead to replace my 80-200 f/2.8L and 100-400 f/3.5-5.6L. Add the 120-300 and I'll have a very fast mid to long put a 1.4x on it and that covers 400 at f/4, add my 500 f/4.5 and I think I am set for Jellystone this fall.
The other options are too expensive I think:
70-200 f/2.8L IS II and 300 f/4 or 300 f/2.8 and either a TC or 400mm of some sort, and I am talking $$$$. Though, I have to say everything I've seen of the new 70-200 is awesome. I kind of want one, but the 70-200 f/4L I have at home will probably do just fine.