USB Cell Phone Booster

I’m looking for a portable way to boost cell phone signals when I’m in places that I only get a bar or so of reception and can’t maintain a connection. By “portable” I mean I’ve been considering going as far as mounting an inexpensive repeater in my Jeep. ThinkGeek sells a “USB Cellphone Booster” which looks intriguing, and I’ve seen other versions of it specifically for in-car use. Has anybody used one of these? Do they work? They’re out of stock, which is probably a good sign.