Darwin is open source but is there anyone here who has actually used it or got a bootable usable machine from it? I mean, can you even run X on it successfully? I never could (didn't try very hard mind you).

The release of Darwin isn't a massive thing I don't think.

There have been several independent distributions of Darwin. The current one is Pure Darwin. http://www.puredarwin.org

There have been several independent ones because they all fail. I don't fully understand the dynamics here, but the inability of the community to create a usefully working OS out of all that open code for over a decade speaks volumes.

I wonder if it is because it never gained enough intertia when Macs are good enough with the Mach kernel etc.? I wonder why the varieties of Darwin didn't catch on?

I think a free software operating system binary-compatible with OSX drivers and source-compatible with OSX applications (through GNUStep) would be an exceedingly compelling product. Which is why people keep trying to make it happen. I don't think it helps that you need XCode to compile a lot of the source that Apple releases, which means you need a Mac.

Isn't WebKit a fork of GPLv2 code?

As with everything in technology, it's far more nuanced that that. See podcasts and articles featuring Don Melton for reasons why...

LGPL. But, it's still a core Apple technology. WebKit is far more popular than KHTML.