Figured more of this type of information will help future Model S owners ...
I drove 154 miles today (2 hours, 40 minutes). Used 38.9 kWh of battery (253 Wh/mi).
When I left, the Roadster indicated 189 Ideal Miles (IM) and 130 Estimated Miles (EM) (of range remaining). When I returned, I had 22 Ideal Miles and 19 Estimated Miles. This was all in standard mode. The first half (literally) of the trip, I drove 60-65 MPH to be safe on the range. When I came back, I knew I was doing OK so I drove 80 MPH as much as I could. This was with the top off and no HVAC, 80 degrees F ambient.
I "used" (189 - 22) 167 IM to go 154 actual miles. And a lot of that was well over 55 MPH. My 130 EM initial range was based on previous aggressive driving and using the A/C and so not terribly useful for the trip.
During the trip, I watched both IM and EM. After having driven 40 miles (that's what the EM is based off of -- your last 40 miles of driving) at 60-65 MPH, EM and IM were within 10% of each other, so I knew I was doing well. I knew if I continued driving like that, I'd have a total round-trip capacity of 170 miles (and I knew my trip about was 150). And guess what -- I had 20 miles left when I returned -- even after coming back at 80 MPH. And if something weird happened, I also knew I could go into range mode and "acquire" another 20-25 miles of range just in case.
Here's what shows up on the screen in the Roadster. The Model S stuff will likely be even more thorough.