I purchased the $25 for 500mb and went through all kinds of trouble trying to get it to work. The plan works with new ATT equip (GoPhone), such as the Android or Apple products, but you cannot use it with an older device as a modem.
I never did find out if the data plan for GoPhone was only media net, but I think you can use regular data if you are using one of the fancy new devices.
ATT's website states support for any device bearing ATT branding, however this does not include GoPhone I am told. I get the impression that ATT is more geared toward Facebook Twittering with Iphones Androids than it is to letting me use 500mb of data to do my shool work. Try NetZero. $5 a month for 56k dialup - doesn't work wirelessly though.