It is not.
You need iTunes and a platform that it works on. Windows or Mac. Linux won't do.
Files in formats that are not forethought by Apple will have trouble getting onto iPad as well. In other words, forget about using it as just storage.
But wait, that's just the good half of the story.
The bad half starts when you need to retrieve the information from it.
Either you can't at all, or you can only in a crippled form (two words: "optimized photos").
Homework for the inquisitive reader: find a way to back up the content of your iPad without paying Apple for their services.
So now tell me: why can't I just mount(8) the damn thing, again?