Ask Slashdot: Who Has the Best 3G Coverage In California and Nevada?

New submitter derchris writes "We will be on vacation in the U.S. next month for about 3 weeks. We are going to do a road trip between San Francisco, Las Vegas and Los Angeles. To not use roaming for data, and get a heart attack once back home looking at the mobile bill, I was looking at so called 'MiFi' devices, portable 3G Wi-Fi hotspots. As far as I know, more or less all of the U.S. carriers have such devices available. But as I'm not from the U.S., I have no idea what would give me the best 3G coverage in the areas we are travelling. Another question would be whether I can buy one of these devices off eBay, and use it with any SIM card. Let's hope there are users available who could give some advice on this topic."