i used to use them extensively. unless something has radically changed, they cost nothing in the svn repository but when you pull them down, you have multiple copies on disk which takes double the space because of the way a branch appears like a separate folder in your working copy. am i missing something?
Your working copy has a URL. You can switch it to another URL (a branch/tag) using "svn switch". It only transfers the differences between the two.
You can have two separate copies checked out, but you don't have to.
It is interesting, I have heard several times the comment about svn branches being expensive. But, that is not true, they are checking out the whole branches directory!