I have 9 days left in my billing period and my Verizon iPad mini and it reports cellular data usage of 422 MB, with 98 MB being System Services. The mini is on WiFi 95% of the time, except when I'm walking to/from school (and I don't carry it every day) carrying it or the WiFi networks I connect to are acting up.
Speed: I'm in Manhattan and I got 5.35 Mbps on T-Mobile LTE, compared to 5-8 Mbps on my AT&T iPhone 5s and 11-21 Mbps on my Verizon iPad mini.
Price: The rates mentioned in the press release apply only to voice customers. If the iPad is your only T-Mobile device, then you're looking at:
One time passes:
500 MB for 1 day - $10
1 GB for 1 week - $15
2.5 GB for 30 days - $30
Recurring (30 day plans):
700 MB - $20
2.7 GB - $30
4.7 GB - $40