Judging from our experience, you should probably place the Car Kit on the left side of your windshield so that it does not obstruct your view. Also, be sure to place it close enough to you so that your callers can hear you better.
Of course, the real value of the Car Kit is that it is partially solar-powered. According to LG, 2 hours of sun will get you about an hour of talk time, with a maximum talk time of 16 hours. This seems pretty decent, especially if you live in an area with plenty of sunshine and you tend to drive for many miles at a time. If you live in a foggier area, though, this might be more of a challenge. Also, it would probably be better to leave it in your car when parked to really make use of the solar charging, but we don't know if it's wise to leave a gadget such as this in plain view of potential thieves.
You'll have to charge the device the old-fashioned way to get going. The Car Kit comes with a regular AC adapter as well as an in-car charger. We paired the HFB-500 with the Apple iPhone 3G and it worked great. Though callers still thought we sounded as if we're talking from a speakerphone, we could hear each other just fine, perhaps because of the echo cancellation in the device. We even made a few calls to an airline's automated phone system to see if it could recognize our voice, and it did.
Features on the HFB-500 include the typical answer and end calls, mute, call waiting, last number redial, and the capability to connect to two devices simultaneously.