I love playing with the twin needles! I've used them to sew bindings down and to create "special" effects when playing with threads. It helps to use stabalizer under your stitches if you don't want that tunnel effect.
You might want to check with the Brother dealer to make sure your particular machine will accomodate the twin needles before you use it though. I didn't know if you bought the whole shebang from a dealer or individual. Have fun though!