You can configure it to open the drivers door only, or all four doors. It is
done the same way as on the keys, just press the button until the
corresponding LED lights up. Concerning reliability: I always take a normal
key along, just to be sure, didn't have to use it on three cars in two
years. I got used to it pretty fast, wouldn't want to miss it. The missing
autolock is the only problem in my opinion. MB doesn't do it for security
reasons, so nobody is locked in the car inadvertently.