I've just started using canonical tags for some pages on one of my sites. I've had a few pages where external (or occasionally internal) links were incorrect and had added extra parameters to incoming links. E.G. www.example.com/pagename.html had been linked to as www.example.com/pagename.html?=garbage which still displayed the same content.
Too early to tell if there are any issues.
I'd setup a 301 redirect, but I've found my mod_rewrite skills to be lacking and the danger of conflicting with some other redirects was too great.