I downloaded the Amazon app earlier this week. First download was the free Angry Birds Rio. I was not asked for my credit card #, perhaps because they have changed that policy. A lot of people complained about it.
I guess they need the phone number so they can send the app to your phone? I "ordered" Birds on my PC (cause I was on the site to order other stuff) and it was sent to my phone. Haven't tried accessing the store on my phone yet.
It seems as if free apps of the day show the regular price (with a line through it) as the default.
I have only downloaded 2 apps (both games). I deleted Birds after it froze my phone twice. Even though a lot of reviews on the Amazon complained about similar problems, it may have been because my phone (Hero) couldn't handle it.
A problem I have is that Amazon says you have to be logged in to use any downloaded apps. Why? So Amazon can collect usage stats? And if you uninstall the Amazon Appstore app, you can no longer use any of the apps you downloaded.
I am not sure what advantages there are to getting apps via Amazon. Convenience if I am already on Amazon? Maybe it is part of their "one stop" approach. And apps are a big thing now. Guess Amazon want to get in on the action. Fair enough, I guess.