The interface is okay, nothing too special, looks a little like the old Android Market or old Appbrain.
It is linked to your Amazon.com account, and has its own update screen and download screens.
My only objection to the download is it doesn't integrate with anything else. If you download an app in Amazon Appstore, Appbrain / market will see the app being there, but obviously won't update it as it has a separate "paid licnese checker" API. This, Appbrain may keep wanting to update it, then you realize you didn't pay for it so you can't update it through Appbrain / Market, but must go back to the Amazon Appstore. The search and filter interface is also not as good as Appbrain's, but that's almost nitpicking.
Definitely worth a try, if only for the free apps.
