I would get a gps. I have seen too many cue sheets with errors to totally rely on them. And at night, it's just too stressful to try to follow a cue sheet with no backup.
It really depends on the promoter and how well they do quality control on the cue sheet. If there was no pre-ride or it was done by people who used a gps, give in and get a gps. I have also seen major problems with cue sheets if the person that did the pre-ride knew the course too well and didn't have to use the cue sheet. I like to follow a cue sheet and if a promoter releases one soon enough then I will use street view to do quality control on it.
My 830 will run for something like 48 hours on battery saver mode. Particularly if I don't display the map. I generally just use an external battery and display the map though.