Changing the battery in the Smart Key system yourself is not recommended, according to the manual, because of potential for damaging parts of the system inside the key.

I keep my Treo 650 and my TCH's Smart Key in my purse and have experienced no problems with the Smart Key system as a result, though I do keep the keyfob and the phone in separate pockets in my purse. I just walk up to the car and pull the door handle and it beeps twice (not the vehicle horn, just a little beep-beep) and unlocks. It is very convenient when I have my 2 year old son with me, because I can just open the front passenger door, drop the diaper bag on the front passenger seat, open the rear door and put him in his car seat without fumbling around for car keys or having to press the door lock switch.

I do believe you can have the settings changed so that all of the doors unlock when you pull the driver's door handle, but around here I would not want it to do that. If I am alone I want just the driver's door to open. If I have passengers with me and I need all of the doors to unlock for some reason (it's raining and we're all running for the car...) I get the keyfob out and use the button to unlock all of the doors.