There's really no better way to describe Onavo other than a must-have app for any and every iPhone user on a data plan. I'll go a step further: I think it's the very first app one should install. Why? Because Onavo shrinks your data usage (and thus, your bills). All you need to do is install the free app and you're done. The app will then run in the background and do its thing and all you have to do is continue consuming data as you do today… Surfing the web, emailing, tweeting, using maps, etc.