The Smart Key for your Prius (assuming you are getting an '06 like I have) is a pretty neat feature. Basically, as you come within proximity to your vehicle, it "knows" you are there and will unlock the door as you touch the door handle. Chances are, when you open the drivers door, it will open only that door, but if you open the front passenger door, all of the doors will open. That is the "default" setting.

This setting can be changed by YOU, and it tells you how to do it in the manual (or at the very least, one of us can tell you.) It's not hard. You use the fob and a push button sequence right on it.

I have not had any issues locking my doors with the "lock button" on the door. Just make sure you aren't doing it by grabbing the handle, and the car won't confuse you with trying to unlock the door.

Additionally, the "Smart Key" is also the ignition "key" however, using it in a traditional manner (actually inserting into the dashboard) is optional. I leave the fob in my purse, jump in the car and hit the power button. No key insertion required.

As for the bettery in the fob, the battery is user-replaceable. And the instructions are definitely in the manual. I just read how to do it the other day... Takes a standard Computer CMOS battery (button cell) that you can find at your local drug store, or worst case scenario, your local Radio Shack for a few bucks.

Good luck with your new car!