I have been using the attachment for about a month now and enjoy it.  It is able to froth and steam various liquids for use with the espresso machine.  The attachment makes it easier to add water and steam since it all uses the same base.  Another add on is being able to add milk and having it steam and froth that as well, although that is rather difficult to understand.

I would say this is a nice add on, but not a must have for the espresso machine.  While it makes it easier to use, it doesn't seem to do anything new or special that couldn't be completed using simply the espresso machine.  It is just an ease of use.