I use a Verizon PCMCIA card. I get a special deal through my employer for the unlimited data plan. It's normally like $80 per month, I pay like $30.... But there are hundreds of us in this company using their cards/service, hence the steep discount.
It's pretty slick, works most places, though if you don't have signal, you are not on the internet. It's more secure than WiFi, but only really practical if you can justify the expense vs regained productivity. I'm lost without mine to be honest, and I travel every week.
I can very easily log over 100meg of bandwidth a month- depending on how much I use WiFI, or if I'm not travelling as much.
It allows me tio be very responsive to peers and customers, and allows me to get work done during the day that I'd normally be stuck with in my hotel room.