Depends on what kind you're talking about. The cheezy stickers you put on the back of your battery are crap. However, I'm interested in getting a roof antenna with amplifier for my cell, as my neighborhood is in a dead-zone valley (just my immediate neighborhood; 500-1000 feet in any direction and you get service, go figure!). They work, but they also run you $200+ depending on configuration. I haven't looked too closely into it due to cost.
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Or I could break out my old Motorola bag phone; that thing gets reception even in my basement, haha ...