The REx was designed to be a last minute, keep me going device. But, say you were planning on going into central London with its fees, and wanted to ensure you had enough battery capacity to run around while there, you would want to use the REx to try to maintain your battery capacity until you got there...so, while the REx will come on automatically if your charge level gets low enough, they provide a means to turn it on prematurely, as long as your battery is below 75% (that's so that there's enough capacity to allow the thing to run long enough to warm up and not overcharge the batteries). If you let the battery capacity get too low, the car will start to go into self preservation mode, shutting down some things (like lowering the max acceleration and speed, minimizing the HVAC capacity), and if you continue to exceed the capacity of the REx output and lower the battery capacity, it will eventually shut down. In general, it has enough capacity to keep you going if your speed doesn't exceed about 65mph unless you've got the heat on max or climbing up a long grade.
When running on the REx, your MPG is only about 34 (US) mpg. Averaged over the EV only mode, it's still high, but if you need to use the REx, it will not be that high. Also note, the UK does have a reasonable amount of DC fast chargers out there, so if you can wait about 30-minutes or so when the battery is low, you can achieve at least an 80% charge or so depending on the temperatures and overall situation. That's assuming the thing works when you get there.
The car is fun to drive, and people do take them on long trips, but once your battery is down, refilling the REx once an hour or so can get old. It works.
The REx is s serial hybrid...all of the other hybrids out there right now are parallel hybrids. That means that the motor only drives the generator, never drives the wheels directly. All of the others can drive the wheels and have an electric motor that can also drive them, sometimes at the same time...that does not happen to the REx. Consider that it's only about a 34Hp motor...think of a car with only a 34Hp motor...once the batteries are low, that's all you have to keep you going...it's not much. Luckily, just cruising, the car doesn't need all that much, and it can maintain.