I'm liking Pupil a lot. I use it not only to change the resolution on my rMBP, but also to set the resolution on an external monitor, which my rMBP often doesn't set correctly when I plug it in (apparently it can't always cope with the daisy-chained Thunderbolt to DisplayPort connection).
Unlike other apps I've tried, Pupil doesn't crash when my external monitor appears at the wrong resolution, it only displays the resolutions I want and not the zillion possible resolutions, and it allows me to put a name to each setting.
I'm knocking half a star off because currently Pupil can only display settings for one monitor at a time. I've been in contact with the dev who says he's planning to add multi-monitor support, and when that happens I'll update my review to 5 stars.