I remember the launch of the cube. I remember thinking "awesome computer, crazy price". It was like a super powerful consumer version of the Powermac G4, an upmarket iMac if you will.
I worked at an Apple reseller at the time, but I can't remember ever selling one! It was the 'halo car' without a doubt - the show of technical prowess without being the thing most people buy. The PowerMac G4 at the time was such better value (slots, HDDs, ZIP drive built in, dual processors available, etc).
It kinda saddens me that the new Mac Pro is like the cube of Mac Pros, but they killed the other line to release it! Lucky Apple didn't kill the PowerMac G4 when the released the Pro range or they would have been in trouble with their biggest markets back then (creative pros, people they don't seem to care about now).