I'll have one installed in the Subaru. Though I rarely use it in the city, it has one incredible use that I would have a tough time living without now:
Cold Temperature Start.
This feature allows you to set the starter so that it will start the car at regular intervals (usually 2-4 hours) depending on the ambient temperature. For those of us that might leave a car at a trailhead in the dead of winter for days, this feature is a LIFE SAVER. Literally! We've come out of the backcountry in -40C and had the car start despite no plug-in for 3 days or more.
And for that feature alone, I will install one in the Subaru. Can't be done in the Mercedes thanks to the factory security. And yes, I know about bypass systems, but they don't work on the Mercedes SmartKey.