What about Direct3D... I heard it's API is really crappy, so is the documentation. So, it could be hard to learn it, true?
False. It's actually the opposite; OpenGL's API hasn't changed much since the 80's and the documentation is practically nonexistent (in fact, if you go to the OpenGL website, it says "The two essential documentation resources that every developer should have are the latest releases of:" and then gives two links to Amazon books you need to buy).
It's much harder to learn to use effectively, since the API no longer reflects much of what happens in modern hardware, and there are multiple paths to accomplish most tasks, the right one being non-obvious or even changing depending on which hardware vendor or driver version you're using.
That said, if you're targeting any platform other than Windows, you don't have much choice but to suck it up and tough it out anyway.