Mobile data usage monitoring (and data compression) apps.

Just reading about this Onavo app which monitors your mobile data usage and can actually reduce your usage by compressing data (through a cloud)
as you use it. This sounds ideal for parents worried about their teenagers going over their data limit and getting stung with big bills.
Anyone use it or tried it? Does it help in the way I've suggested?  Is there anything similar or even better I should look at ?