For a personal web design project I am working on I needed a way to display the total number of approved comments (or in my case, reviews) a user has made. I thought this would be a piece of cake but there is very little documentation on it. So here you go:
If you want to find the total number of comments a user has made from a comment ID you can use this:
//count approved reviews for this author
$args = array (
'user_id' => get_comment(get_comment_ID())->user_id,
//more args, e.g. 'status' => 'approve',
echo count(get_comments( $args ));
I used this in a custom comments callback display function. Depending on what your needs, you might want to add some other arguments for get_comments function.
Hope that helps someone.