The Range Extender gives me peace of mind when riding with the many cycling clubs that I ride with. I don't have range anxiety as it adds around 20 miles, based on my typical use. I burn around 2-2.3% of battery capacity per mile. So, with this battery adding 50% and once you subtract 5% for battery health, yielding 45% and divide that by 2.3% you'll find it adds (for me) around 19-1/2 miles range.
When combined with the internal battery, my typical ride of from 45 to 56 miles is not a problem, range wise.
My wife's battery failed after 2 months and Specialized had a local shop exchange it the same day. Not bad....