I recently used Verizon's mobile broadband service when I was in Europe for two months. I signed up for the 100MB plan and bought the modem, because I envisioned that I might keep the plan when I returned from Europe. Verizon store personnel warned me that sending large files (e.g., Excel files) would eat up significant MBs. As such, I avoided sending any Excel files while I was there.
Approximately two weeks into my stay, Verizon abruptly cut off my service. When I contacted them, they indicated that I had used almost double the MBs under my plan. Customer Service then explained that accessing websites could use substantial MBs, which is probably what happened. My bill for the service was astronomical. From then on, I used the service only when I couldn't access a wireless network. Verizon took a "buyer beware" attitude and gave me only a very small credit against the bill.