I actually like the side buttons and find them useful for browsing. To the point that I've been actively annoyed at the last few mice I've gotten (that don't have these buttons).

The problem I kept having is that they would trigger when I was just picking up the mouse—they were very sensitive—and close a page I was using (which was especially a problem if the page had dynamic content of some sort). Also, the default button action ("back" in the browser) wasn't all that useful to me, as my browser workflow usually involves open-in-new-tab and closing tabs rather than a lot of back-and-forth.