I use Sprint and the Sierra 595 for my labtop with 3 TradeStaion workspaces open. Works really well... data is quick enough for my purposes (i trade 5 min bars).
This is true in Verizon's case. I've been mindful of it so I avoid getting, for example, streaming CNBC Plus video.... (though I've done audio only for a few hours here and there w/o getting close to the limit).
i have the sprint card as a 3rd backup plugged into a wireless router called junxion box and it has served me well for about one year now. its pretty good better than nothing.
I use VZW EVDO rev a when I travel. I have daytraded on it with my full setup. It's not ideal compared to what I have in my office but it does the job.
I have my execution and separate charting software up for 12 hours at a time.