The AT&T international unlimited data plan costs US$64.99 per month. If you are an occasional international traveller, then buying a local SIM with data is probably more worth it.

For frequent international travellers who spend a lot of time outside US, I personally think the AT&T international unlimited data is better. As soon as you land, you are connected. You don't need to worry about buying a local data SIM. More importantly, there is no other operator anywhere in the world that offer international unlimited data up to 5 GB with no throttled. Some mobile operators in Asia offer unlimited data roaming for like US$15 per day but you get throttled once you reach 250 MB per day.

This plan has been discontinued by AT&T. So, if you have it and travel frequently, keep it!