I work from home, so I have the advantage of not needing 240V charging in my garage. There was already an existing 20A 120V plug in my garage. I get 5mph charging. On a busy day, I can drive 40+ miles running errands and helping out my wife with the carpool. If you have a 20A 120V plug in your garage, do the math to see if you even actually need to install a 240V plug. After overnight charging, I typically start the day with a full 90% charge unless I've done a lot of driving like on weekends, or coming back home after a long roadtrip. I plug in the car whenever it is parked in the garage. Even if I am home for only a couple hours before I have to go out again on another errand, that's 10 miles of range while the car is just sitting there in the garage.
So here is some math I did before buying the model S. I drive approx 15000 miles a year. Worst case scenario, that's 3000 hours of charging at a rate of 5mph. I was skeptical before I bought the model S, but after a month of ownership, I don't think I'll need to install 240V charging.