The last two months finally saw rapid progress in the development of the Search API module's Drupal 8 version. Acquia generously agreed to fund all available time for both Joris Vercammen (borisson_) and me in December and January to work on this port and related modules (especially Facets).

What we did

With this backing, we were able to make a lot of head-way and got a lot of large blocking issues out of the way, among them a overhaul for the fields UI, some necessary major internal refactorings and most of the Views integration. All of this is now baked into the new Alpha 12 release, created today. Over the next couple of days, we will then also create releases for the other related modules with a working D8 version: Facets, Search API Solr Search, Search API attachments (Alpha 2) and maybe also Search API pages.
That way, we should be able to avoid a confusion of versions and conflicts for any users interested in trying out the current state of work of this module suite, or already starting to build a new site using them.
Going forth, we will also try to keep this system of creating a set of compatible releases for future Alpha versions.

As noted in the release notes, though, be careful when building sites already with this module version, as there will be no upgrade paths until Beta and some changes until then are still likely to break the storage structure (and would thus lead to loss of configuration, unless handled correctly). Also, this release (like all other non-stable releases for any module) will not be covered by Drupal's Security Team, so any discovered security vulnerabilities would be reported, worked on and fixed publicly.

That being said, though, one of the greatest improvements in the module's D8 version, at least under the hood, is it's vastly improved test coverage. That, along with Drupal.org's automated testing, enables us to be very confident in each new feature we add and each bug we fix, thus also improving the maintainability and speed of feature development in the future. And it hopefully makes it much less likely that any major bugs go unnoticed for long.
But there are also lots of improvements visible right on the surface: we carefully reviewed all major encountered problems and pitfalls with the module's D7 version and worked to make the new D8 version another large leap forward to support as many search use cases as possible, while still becoming much more user-friendly than the D7 version – probably one of the largest points of criticism overall.

Still to do

So, how does it look for the further path towards a stable D8 release for the Search API (and, subsequently, for its numerous add-on modules)?
Currently, there are no immediate plans for further funding, so while I will of course still work on the port whenever I can, the pace will necessarily slack down a bit again. I also neglected maintenance of my various D7 modules in the last months, so there's also a lot of catching-up to do there. (Incidentally, a great way to help this effort if you are not comfortable with D8 yet: just go into any of my modules' issue queues and try to answer support requests, reproduce or fix bugs, test patches, etc., there.)
However, while there are still a lot of beta blockers left, most of them are relatively minor compared to the ones we now resolved, so I think a first Beta release in March should be within reach. Then it will be a matter of determining the MVP for an initial stable release and working towards that – but I expect a much shorter period for Beta than it has been for Alpha, maybe only a month or two.

If you want to help with this in any way, please either ask around in the Search API IRC channel (#drupal-search-api on Freenode) or send me a message – or just jump into any issue and get cracking!