I ran into this problem as well. It’s not that it never works, it’s that it will always return false until after comments_template() is called. I’m not sure why this doesn’t use the same method as comments_number(), but it doesn’t and it sucks. What you should be able to use instead is get_comments_number() which returns the number of comments.
I know this was posted a month ago, but hopefully it will save someone like me some time.
