I have 4GB of AT&T for phone (Galaxy Note 2) including tethering (paid by employer) and 2GB for an iPad per month(paid by me)

When I travel domestically and a conference or Hotel does not provide free WiFi or the connection is unreliable/slow, I can burn the whole 6GB a month. If I stay at Hilton, Marriott or somewhere with free and decent WiFi, or not traveling then I could leave a lot of data on the table at the end of the month. Since AT&T has turned on LTE in my neighborhood, and its faster than my DSL, I plan to clean out my remaining data at the end of the billing period.

For International use, I usually get a country specific pre-paid SIM.