This is really a design-specific question. I personally, from a usability and accessibility standpoint, don't think having the spinner replace the text on the button is a good choice. I do think that users generally like to know that something is going on behind the scenes, I personally prefer a slightly transparent overlay on the page with the spinning icon in the middle of the screen if it's the final action on a page. If it's one of many potential actions on the page, I'd put some sort of indicator in that area, but not necessarily on the button itself.