AGK is certainly stable enough for Android development, I've got a couple of apps in the play store.
3D has been a bit "iffy" up until recently (and still lacks a few features that made DBP so awesome). I've made a couple of 3D Windows Apps (that I haven't released), and they work fine, but I encountered multiple problems during development, having to implement workarounds.
For 2D development it's absolutely fine
My current apps are ad supported (admob) and it works really well, using both banners and full-screen ads. I've heard of issues in the past with the other ad providers, but I think these have mostly been fixed. Admob works fine in AppGameKit with no issues.
As for the publishing of an app, it's now easy, simply choose "Export APK" and you're done.
Also, as CJB suggested, there's loads of stuff in the showcase that you could try out to get an idea of what can be done in AppGameKit, and on the AppGameKit Showcase forum: https://forum.thegamecreators.com/board/48