Anyone using an international data sim card that they can recommend?

I've tried Worldsim and Sim4globe with very poor results. I tried both using a iPhone 5S and kept getting a PDP Authentication Failure message followed by the phone not receving data.

I'm looking for a pay as you go solution. T-mobile is awesome for worldwide data, but unfortunately they only offer a $75/month plan. They're service isn't great in the U.S. and so I'm looking for a part-time international solution. Don't need talk just data.