I pre-ordered the MC-11 adapter at B&H about two weeks ago. Was informed via email on the 21st that my order shipped and is now in transit - with the expected delivery date of 4/27 by UPS. So it does appear that those who pre-ordered the MC-11 adapter are being shipped out now. Yesterday I checked and it was listed as being "in-stock", but currently it's listed as "back-ordered" for both the MC-11 lens mount adapters - as well as B&H's MC-11 adapter and compatible Sigma Art Lens packages.
I currently have a Sony a6000. I also just acquired a used (mint condition) Sigma 17-70mm f2.8-4 DC OS Macro Contemporary for EOS Mount lens for less than $200.00 on eBay. Even adding in the cost of the MC-11 adapter ($249), it's still around the same overall price I'd have to pay for a new copy of this lens without the adapter. And by now owning the MC-11 adapter, I can consider buying one or two of Sigma's great Art Lenses for my use on my a6000 that have nearly the same AF function capabilities that the Sony/Zeiss E-Mount lenses offer, but at a substantial discount in overall cost.
If the Sigma MC-11 is anywhere near as good and versatile as some of the preliminary testing with various Sigma/Canon lenses seem to be indicating, then this adapter may really be a game changer for current owners of Sony's E-Mount cameras - instantly broadening our available of quality lens options at more affordable price points.
Kudos to Sigma!