I opted for Matrix connection in my last travel to US. The reason why I took this connection was I could be in touch with my family until I took a phone connection there. I called up the Matrix customer care and they promptly send an executive with a new SIM. I had to submit my passport, visa xerox and fill up some forms. The whole process took not more than 30 min. The executive told me that the number would be activated once I landed in US. The matrix sim worked with AT&T network and I didn’t face any problems. However the charges are very high. After end of every billing period, a detailed bill was emailed to me and my credit card was billed.
After I returned a representative collected the sim.
My advise to people taking the connection, use it only for emergency purposes and get a local connection. It would be much cheaper.