I used to have 500MB on my plan, but only used about 250MB per month. Wifi at work and at home, so what went over 3G was mostly checking my mail/facebook/twitter while away from home/office, and occasionally using Google Maps (which accounts for a big chunk of the 250MB!).
That said, I avoided doing bandwidth eavy things like using the YouTube or Vimeo apps on 3G, and my provider disabled tethering and Skype. These days I have unlimed bandwidth, so I often listen to talks on YouTube while commuting. Usage is accordingly much higher - upwards of a gig, but that's really the only difference.