Some pieces of the backup can be viewed, not all of them, and that goes for only some files and such. As for attempting to manipulate them, you can, but your iPhone won't read them properly through iTunes since iTunes won't recognize it as what it backed up. As for where they are stored, they're stored in the following directory for Windows (example) for the most part
C:\Users\Jon\AppData\Roaming\Apple Computer\MobileSync\Backup\