I've only used a jelly roll one time; it was okay but I wouldn't purchase one. I felt too limited by the color choice--some I would have liked more of, some I didn't use. I've never bought squares or or charms for this same reason.
I rarely buy less than a yard of fabric at a time. This is just me though, because precuts seem to be very popular. (I also fear cutting wrong and not having enough fabric.) I like coordinating my own colors (even if they're not great) rather than using someone else's choices.