(To be clear, I know there are good reasons for this, and I'm not trying to imply that a lot of work hasn't gone into what has been released ... but I can't help but wonder if there's no longer any hope of it catching up with (semi-closed) Darwin.)
I have used OS X full time for 7 years now, and I love it, but I never thought the kernel had much to recommend it.
Giving what the mingw project has contributed to being able to build Windows binaries on Linux, I'm pretty sure it would be possible to do something similar for OSX, but since so much of it is already open source, it seems a waste to discard the exact code that OSX is running
Also, having alternatives is always interesting. I was also interested in making the core of OS X† have some form of FOSS liveliness, but I've been quite disheartened about OS X and openness (however limited) since this[0] happened.
[0]: https://github.com/lloeki/xbox_one_controller/issues/2
† I do believe that FOSS is very important, but I also recognise the critical effect of closed source innovations, and believe that a delicate balance and synergy between both aspects can produce fantastic results.
I'm glad to see someone took it over and has a goal of making Darwin ISO files. I hope it also gets an OSX themed skin for whatever Desktop GUI they decide to use with Cario Dock or something to look like OSX.
More to the point, I'd expect the first-milestone goal of an "OpenDarwin" project would be to be able to install and run OSX's Server.app and act as an OpenDirectory master, NetBoot Restore manager, etc. That proves compatibility, and is a useful thing all on its own (because, among other things, it would mean being able to manage OSX from non-Apple hardware†.) Doing a GUI first? Crazy talk!
† ...without spending weeks tearing your hair out trying to conform a Linux LDAP+Kerberos+whatever server to OSX's idea of what an OpenDirectory controller is supposed to look like. I tried, I tried so hard...