Definitely agree. Generally, I think good hacking music is unobtrusive (doesn't become a distraction), and somewhat repetitive (causes me to maintain focus). I hate a lot of electronic music because it's often too formulaic, but when it's right for hacking it's oh so right.

Some of my hacking favorites: Aphex Twin--Selected Ambient Works 85-92 (the one essential for hacking IMO), Sigur Ros--Takk..., The Field--From Here We Go Sublime, Linstrom--Where You Go I Go Too