I started a shared data pool plan for my family and my brother's usage was estimated at about 2GB per month. A couple weeks into the billing cycle I checked usage and my brother had used MORE data than the other FIVE of us combined, and was on track to use over 5GB!
I was a bit disappointed originally that I wouldn't get shared data when I switch my extended family over from AT&T, Verizon and Sprint (seriously, all 3 other networks!), but I soon realized this was a huge boon - because T-Mobile doesn't charge overage for data - only steps down when you reach the limit (i.e., data limit is for "fast data" - i.e., LTE or 4G, but the 2G is unlimited). So my post-grad cousin who has erratic data usage doesn't impact my bill or my speeds. My bills never change month to month - been that way for over a year now, and it's an awesome feeling (remember having to try to parse a cell-phone bill wondering why we paid $130 this month and $80 last month for the same line).
Oh, and I'm spending the same on phone bills - except now I have 5 lines where I used to have 3, and all with better data and reception than before.
Lesson: don't trust shared data - it's makes each line on your plan a liability for increased data costs on your entire plan, all possibly due to a rogue app.