Let the hive mind of Engadget get that for you.
"As someone who doesn't reside in the USA, I was wondering what would be the best way to get internet for my computer in the US for a couple of weeks? If it were Europe I know I'd look for some prepaid data. Is there anything similar offered by American carriers? A MiFi or a data SIM that I can tether from would work, but I'm trying to maintain a tight budget. Help!"
For a handsfree Bluetooth speakerphone that you leave in the car at all times, I almost prefer a solution tethered to the cigarrete lighter, without any internal battery. Or at least one with a really *weak* Bluetooth antenna/transceiver.
Otherwise, even when you're at home or office and your car is sitting in the garage, your cellphone could still be paired to the handsfree kit in the car if it's inside the 30ft Bluetooth radius. It's annoying to answer a call on your handset and not hear anything because your caller's voice is being sent to the car (and your caller in return hearing silence from inside the car instead of your voice). That is, until you realize what's happening and have to manually disconnect the Bluetooth.
Whereas if the BT speakerphone does not have a battery and only relies on your car's 12V power, then you can be sure that when you turn off the car it will also turn off and unpair with your handset.
Alternatively, a shorter Bluetooth radius (say 10ft or less) would ensure that you won't still be paired when you're out of the car.
Otherwise I do agree that a solar charging solution would be a good idea. The only issue I see is that many handsfree kits are designed to be clipped to the visor where they're right above your head and very close (to minimize background interference). Having a solar charge means you have to put the device on the dashboard which is farther away from your mouth (and closer to engine/road noise), or perched on the windshield glass which looks hokey.