I have to admit I'm on the fence on this one. I've been taking them off and putting them back on about once a month trying to decide what I like better. The naked wheels look nicer, but I do a ton of freeway driving and the extra efficiency is much appreciated with them on.
One thing I will say is that the tire sidewall height looks appropriate with the aero covers on, whereas (to me), the tire sidewalls look too tall when the aero covers are off. Like an 80's car before lower profile tires were available. Maybe if there was a 19" version of the aero wheels...