Here's a good writeup on Ars about the Amazon Android Appstore:

http://arstechnica.com/gadgets/revie...-in-places.ars

The bottom line, it shows a lot of promises. Here's a quick rundown:

Pros:
+ Integrates with Amazon Account -- very common
+ Seamless purchase experience
+ Functional search feature
+ Good app recommendations
+ Free apps (Angry Birds Rio/Doodle Jump)

Contra:
- missing notification-bar progress for app install
- doesn't know of what's already installed on your system from Google's App Market
- requires app install to use, not pre-loaded -- may be difficult for casual users

All in all, it shows great promise. Amazon has a bit of an uphill battle ahead of it to get new customers, but they are aggressive and have points of entry. Considering their large book repository, movies, and music, Amazon may be considered a one-stop-shop for all of your digital content. Potent.