Recently I got e-mail from some Amazon AppStore worker and he invited me to publish some apps there.
So I tried.
Here is my summary:
+ Android apps made with CocoonJS works without any problem (so you don't have to make 2 versions: Android and Amazon)
+ you can use AdMob too (confirmed by Amazon guy)
+ your app will be live within ~24 hours after upload
+ there is much less competition than in Google Play
+ nowadays it's free
- it's less popular so you can get 10-20 downloads / daily for free app
Is it worth?
I think yes... Let's say that your app is quite good (so only 50% users will uninstall it) and you have 10 downloads daily. After 1 month it will be 300 downloads and 150 installations on devices. After 2 months 600 downloads and 300 installations on devices... And so on. At this point it can generate a few additional dollars.
So I think that it is worth to submit our apps also to Amazon AppStore, because it's very easy and takes only 5-10 minutes (including binary upload time).