It's still way too early to tell how much traction Amazon's Android Appstore will gain. But we know developers have been testing the waters. And it makes sense. Registration is currently free (they've waived the $99 fee), Amazon has tens of millions of customers, and the more places your app is available, the more likely people will be to buy it.
Something that shouldn't be taken for granted is the actual experience of uploading applications to the respective markets. Amazon and Google are pretty even in what's they're doing. You upload an .apk, give it some metadata (keywords and the like), add some screen shots, and away you go.
Amazon's Appstore has a couple of additional features that are unique to it because Google (still, sigh) doesn't have a proper web portal. You can set the date that the app page will become visible to cusotmers, when you want to start preorders, when you want the app to be available for download, and when you want to discontinue sales of the app.
And beyond that, the developer portal UI is just much, much cleaner on Amazon's side. It's unlikely that'll matter too much to the developers -- after doing all that coding, a sparse (but utilitarian) uploading UI is probably the least of your concerns. But it's nice that Amazon put a little thought into it. We've got larger screenies after the break.
Do developers are pumping apps into the Amazon Appstore. At what rate? Unknown. It's only been open a couple of weeks now And we still don't know when it will open to customers. Stay tuned, folks.