They definitely make a difference! Especially when in TTs a portion of a second can make the difference between placing(so year end points,purse money for that event,etc.). I am getting one for the '08 race season, I can't wait!!( my goal is to be in the top 3 for the cat 4 women for States at the end of the year next year).

Besides being more aero, they make a whistling sound when you are out of the correct aero position, because the wind isn't going over you correctly. So they act as a constant reminder to stay in the most aero position, therefore helping you help yourself.

Definitely worth the 160 or so for one if you are going to do TTs competitively.