Consider your parking locations: If you park your car in a garage or in a secure parking lot, you probably don't have to worry about leaving your device attached to a visor or on your windshield. In that case, you can opt for a unit that can clamp tightly onto your visor--since removing a speakerphone from a tight position multiple times a day can be a pain.

Be aware of flashing lights: The great thing about headsets is that they can blink all they want, and the wearer won't notice the lights while the doodads are on the ears. Many car kits, though, sport blinking lights. At night especially, that blinking can make them extremely distracting--enough to make you want to switch them off. If a constant blink will drive you demented, shop around for a unit that lacks lights or keeps any illumination to a minimum. The Jabra SP700, for example, offers a night-driving mode, in which you can switch off the lights entirely.

Use natural resources, if you can: Some Bluetooth speakerphones are solar-powered, meaning that they can recharge simply by having their solar panels exposed to sunlight regularly--no AC adapter or USB charger required. (Note: You do need to charge the unit when you use it for the first time.) This is great if you live in an area where the rays are guaranteed. If you don't, then such a device loses its appeal.

Buy no more than you need: Some speakerphones let you stream music from your phone and broadcast your calls through your car's stereo system. Such features might sound appealing, but do you really want to pay for them? (The listening experience isn't anything to get excited about, after all.)

Take a test drive: If a friend or colleague already has an in-car Bluetooth speakerphone, go out for a jaunt just to see (and hear) the product in action. If possible, head out at night so that you can witness the flashing lights (if any).