You need a very minor hack to access Google Market. I have a Prestigio Multitab, and the OS on that is just as customized as the Fire. For that matter, the Android on my Sony X10 phone has been even more customized by Sony, and they don't even support it anymore. (IMO, that's the biggest shortcoming of Android devices)
For a non-Android tablet my Kindle Fire is running Ice Cream Sandwich surprisingly well next to me here.
As I said above, consider me corrected.
Will be interesting to see how Amazon develops the forked version. From the article I linked -
It’s not clear yet if Amazon intends to update its fork of the operating system to bring it into alignment with Android 4. Amazon’s changes to the operating system are said to be much deeper than the kind of cosmetic changes that handset manufacturers typically make to differentiate their products. As Amazon’s flavor of the platform continues to diverge, application developers will likely follow in order to reach the device’s audience.