As of this posting, many users have been able to successfully download Google Maps, but some are given a notice that the app is temporarily unavailable.
Apple approved a Google Maps app for download from the iTunes store late Thursday night, finally giving users an alternative to Apple's own in-house maps app. Apple's maps has been a point of contention since September, when the company kicked the Google-supported digital atlas off its platform. When users finally got their hands on Apple's version of maps, users found it to be sub-par.
Source: Ars Technica. Read full article. (link)