I like the alerts on minute, messages and data. But one thing i would like to see is the ability to set alerts based on the estimated monthly usage that is calculated on the dashboard. The ability to set alerts on this number would let me know earlier in the month if my average daily usage is too high. That would give me plenty of time to modify my behavior.
Example: If i set an alert for 900 meg of data and it alerts me on the 20 day of my billing cycle that i am over 900 then i have less then 100 meg left for the next 8-10 days. But if i could set the alert on estimated usage i would likely be alerted much earlier in the month that i am on track to overshoot my budget of 1000 meg per month. That might prevent me from going over, or having to turn off data for the last few day of the billing cycle.