I know I am not taking relativity into account but outside of that it's something like 250 kilograms (~550lbs) of antimatter (and that value again for the matter for it to react to) or the equivalent in other fuels. The only difference is how quickly you burn through it.

Which doesn't sound like a lot but its probably as much or more antimatter than exists in the entire solar system, so if it were the fuel then whoever was using it would need to have figured out a method to create and store antimatter in bulk along with the ability to react antimatter as rocket fuel without destroying the rocket its fueling.