Plain and simple: go to any shop where you can see a 13" rMBP next to a 13" Air. See no difference: great for you! See any difference but not interested because of surfing-mailing-texting routine: still great
Macworld tests seem to indicate a 20% difference on most applications that require some power (iTunes encoding, Photoshop, Aperture import, gaming,...). On top of that comes a retina display, so what's the fuzz about 100 to 150 $?