A really really long time ago, I remember hearing Jared Spool report usability research that said text links trumped images. I mean, a really long time ago. In the ’90s, I think.
I don’t know if it’s the “button” or its placement on the screen, but I seem to regularly get a headache trying to find “edit” links when they are disguised as buttons. Maybe it’s because button=submit in my mind. In any case, Diigo is the latest example.