First of all, you need to accept--these days, anyway--that your conversations over a car kit will sound exactly how they typically would on any speakerphone. Factor in varying levels of interference and noise inside and/or outside the car, and decide whether you can live with that.

To buy or not to buy: Determine whether your hours in the car on a daily basis would warrant a speakerphone purchase. Perhaps you can make do with a Bluetooth headset--unless your ears tend to get sore.

Follow the letter of the law: If you are interested in a windshield-mounted speakerphone, such as a solar-powered unit, find out whether it's legal in your state to drive with something attached to your windshield. (Check the Web site of your state's motor vehicles department or highway patrol.) Windshield-mounted devices should have an alternative installation option--they should be able to attach to your dashboard or sit on it. But remember that dashboards vary in texture and shape from vehicle to vehicle, so your dashboard may not be flat enough to accommodate a windshield-oriented device.

Figure out if portability is a priority: You might want to be able to move the speakerphone several times a day, such as from car to desk or from one vehicle to another on weekends. If so, select a model that doesn't require more than a couple of seconds of positioning. You may want to avoid, for example, an installation scenario where you must first attach a clip to the visor, then align the unit's magnetic plates, and then lock the speakerphone into place. (Undo. Repeat. And so on.)