I look at the G4 Cube as a step on the way to the Mac Mini. For years, a lot of folks were clamoring for a "headless iMac," so they could save some money by using the monitor they already had. The Cube wasn't it. The Mini was.
The Cube, the 20th Anniversary Mac, and (I would argue) the first iteration of the MacBook Air were what Apple-bashers claim all Macs are -- machines that charged a hefty premium for aesthetics. There is a limited market for such machines. On the other hand, machines with smart, functional, and yes, aesthetically appealing design, like the iMac, the MacBook Pro, and the current-generation MacBook Air, iPhone and iPad sell quite well.
Agree, except part of the Air's aesthetics was its extreme portability, an actually useful feature. The problem was then it was far too slow to be even a main road warrior machine. But it was a technological feat just to get it that small in the first place, much less worry about performance.
In fact, the Cube even had some high technology behind the looks. The kind of molding technology to make that case was invented by Apple, and the system was carefully designed to allow chimney cooling without fans. It's not like the standard, stick a fancy plastic faceplate on a box and call it "design."
And, yes, the 20th Anniversary Mac was just for show, a "here's what we can do" vision thing. I was surprised Apple didn't make it a limited edition in the first place.