Are there any developer docs? How extensible is it? How we do integrate 3rd party apps into the wagtail admin?
Clearly this isn't enough and we're working on proper Sphinx docs which should be ready in the next few days.
Extensibility: it's just a Django app, so you can include it in your existing project, or add your own apps / views / middleware etc. There are some hooks, e.g. for including your app into the main admin navigation - see https://github.com/torchbox/wagtail/blob/master/wagtail/wagt... for an example of where a hook gets called. It's a good candidate for developer docs!
The editor and image library is very nice.
I feel you've gone slightly too far in the 'flat and beautiful' direction. Some UI elements lack affordance and I think usability has suffered slightly at the hand of aesthetics.
Finally I still feel there's a case for not completely replacing contrib.admin but that is a complex debate and I can see in your specific case you probably made the right decision. I'd really like to see if I could achieve some of your elegance without abandoning (or reinventing!) contrib.admin.
Best of luck!
There are still a couple of Wagtail features which rely on contrib.admin. We have to decide whether to smarten up the contrib.admin UI or pull them into the Wagtail admin. Is your CMS available somewhere public?
Surprised you haven't looked at it - it's the most downloaded of all the Django CMSes.