IMHO, if you are talking about city pairs like LHR-JFK, FRA-LAX, the proposed .98M Sonic Cruiser doesn't provide much added value to the passengers. When you include the time one needs to travel to the airport, check-in time, time to clear the customs, and time to travel from the airport to your hotel or home, the time saving in the air really is not that significant.
However, if your travel on the Sonic Cruiser involves intinary that bypasses hubs, for example, DEN-CPH nonstop instead of DEN-JFK-LHR-CPH, then the time saving could be significant.
I firmly believe the biggest selling point for the Sonic Cruiser is not the appeal of travelling slight faster for the passengers. A member of Yahoo's Orders group has tried to come up a one-week schedule of an LHR-based airline and show
Sonic Cruiser time saving which translated into 46% increase in distance travelled. I did a simple two-aircraft rotation for a HKG-based airline supporting a daily flight each to Europe and to Australia. The same two daily flights would require three 0.85M airplanes. Once again, it means each aircraft would have a 50% increase in productivity. The 15% increase in speed has surprisingly opened up many more windows of "sensible" schedule which in turn allows airlines not to idle their long-haul planes on the ground. This is the beauty of Boeing's 0.98M aircraft proposal. The hype of travelling faster for passengers is nowhere as valuable as the potential to increase productivity for airlines.