Anyone else on Amazon Underground?

I have 4 apps on Amazon Underground since two weeks. Results are confusing. One of my apps immediately had more then 200 downloads a day, which continued for about ten days. Now things are cooling of. Stats tell me that on avarage around 2500 minutes a week are spend with my apps, which may look good, but results in about 5 dollars revenue a week.
It seems like there is a quick saturation. At first everything is new and free, so people download a lot of apps, try some and then loose interest. The market size seems limited, and only restricted to the United States. I’m wondering what to expect. The company of other apps is good and high level. There are quite some Disney apps, which seems to indicate that there is trust in the earning model by the big players in the field.

What are the experiences of others?

stay away from amazon.
small user base locked OS. Google Play will be better

Yes, it is for me a kind of an extra, as I’m already on GP. In fact, Underground requires that you have the app already in another app store as a paid app. That’s how they can advertise with ‘actually free’. Because, what is the value of an app anyway? Nobody knows. It only has a value when it is for sale somewere else.
Revenue declines quickly. This week half of last week; made 2.50 dollar in a week on Underground. I wonder how Rovio and Disney are doing. I can not imagine that they make a lot of money either. Suppose they do 100x better then me, that is then 250,- a week, for a couple of apps by giving them away completely free.

Probably they are not making as much as in GP, but consider that their games are very addictive and appear on the top, so getting lots of downloads. Probably earning few thousands a week, as Amazon could have some agreements with those big companies to have their games on the store.

After one month, downloads have grinded to a halt almost completely. From 3500 minutes of total use in the first week to 260 minutes last week. My best downloading app, which did 80% of usertime was first on page 7 of the store, now on page 11, (from 55) which is not to bad. So I lost some ranking, but does this explain the enormous loss in usage time? How are these other apps doing? Is there a saturating market, or are the big companies still doing as good as the first day on Underground?
Second problem I wonder about: the canibalism on my own selling rates. I never sold much at Amazon, but considering the downloadrates of my free versions, which are very humble on Amazon, it was not to bad.
But last month I hardly sold anything, which could be caused by the free full versions of my apps on underground.
What would be the angle by Disney and Rovio on this point?