It has been free for a while. I have been using it for about 2 years. I don't think there is any intention of charging a fee as long as Google Docs is free. SkyDrive is also used throughout Windows Phone 7 (and 8, I assume). When you create a document on your Windows Phone device, this is where it gets stored. I doubt Microsoft would start charging those people, but you never know.

You can download the documents created in SkyDrive as standard Office files (.docx, etc.). I would try it out for now and if there's ever the threat of a fee, you can just take your files and then worry about what to do then.