The new plans start with unlimited nationwide calls, unlimited nationwide texting, and 500MB of data for $50/month. Notably, you won't be charged for overages; however, your data speed will be slowed to 2G levels. For an extra $20/month you can add unlimited highspeed data access but you'll still be limited to 500MB for mobile hotspot usage. Alternatively, you can move up to 2GB, 4GB, 6GB, 8GB, 10GB, or 12GB plans for an extra $10/month for each step.
Presumably, we'll learn more about T-Mobile's plans on Tuesday.