Unfortunately, the Test Drive feature isn't available to test all the available Amazon apps-- But Test Drive is a unique, and cool, new way to vet apps before buying.

Few non-official app stores for Android exist, especially if you choose not to "root" your mobile device to free it from various carrier- and manufacturer-restraints, so you really don't have too many third-party options. (Even the popular still uses Google's Market for downloads.)

No one knows whether or not the Amazon Appstore will stock as many apps or better quality apps than the official Android Market over time, but it should at least provide another useful option for discovering and downloading Android software, even if you simply check the stores' Top Free and Top Paid lists each day for potential downloads.

Bottom line: There's no good reason why you can't use Google's official Android Market and Amazon's third-party Appstore for Android to get the best of both worlds.