Pretty much all new cars nowadays are equipped with smart key technology, which has made putting a key into your car's door to unlock it or into the ignition switch to start your car's engine obsolete. Now you have a transmitter fob, which relies on radio frequencies to unlock a car's doors and start its engine. At the beginning, when smart car keys were first introduced, they were mostly used in luxury cars, since it was an expensive technology, and it didn't make sense to implement it in less expensive cars. Some of the most popular and advanced smart key systems include Mercedes-Benz's Keyless Go integrated into SmartKeys, Toyota's Smart Key System, Lincoln's Intelligent Access System, and BMW's Comfort Access.
Vehicles that come with such technology are equipped with computerized systems, and can identify the driver as they are approaching the car. The system uses several microchips and sensors that transmit information to the smart key, which opens the car's doors thanks to a radio transponder chip that is installed in the key. This is a so called Passive Keyless Entry System, which doesn't even require you to take your key out of your pocket or your bag and pushing a button to unlock the doors. Once you've entered your car, you can start the engine by pressing the engine start button.
These are perhaps the most significant advantages of smart keys over traditional car keys. In addition to being very convenient, they make your car difficult to steal. This is because thieves can't unlock and start a car that has smart key features using the traditional method, that involves cutting some wires, then stripping and connecting them. The computer system in the car requires that the correct code from the smart key is transmitted, and if it doesn't receive it, it won't get the car going, no matter which and how many wires are cut and connected.
As far as the downsides are concerned, it's worth noting that there are some added costs that come with owning a car with a smart key technology. If you happen to lose your smart key, it may cost you over $100 to replace it, whereas replacing a traditional key only costs $10 - $15. Plus, you can't buy a new smart key wherever you want, since it's a technology that is trademarked by your car's manufacturer, and you can only get one at an authorized dealership, which means you'll probably waste a lot of time and go through quite a hassle while trying to find a replacement key.
Furthermore, if you forget to turn off the engine before you leave the car, and leave the car in your garage, carbon monoxide will be released and it will get inside your house, which can result in fatal consequences.
To sum up, smart keys can definitely make your car rides more comfortable, but you have to keep an eye on them and try to not lose the keys, since you will have to pay a lot of money to replace them.