I would not call it "useless", but...
...it was a Powerbook G4.
There were a few other things which I regretted buying, however nothing as expensive as the PB and thus got eclipsed in comparison.
With the PB, I think it is a case of the wrong choice. All along I had been using various flavours of Windows and as much as I get frustrated with various nagging issues, I am fairly comfortable in using it. I started my liking towards the usability (and design) aspects of Apple years ago and finally broke down and bought the PB.
And just could not adapt.
Mainly, I was frustrated by the lackluster scrolling behavior (it was very jerky in 10.3,9), no real full screen mode etc etc. There were a few other things too (the usual rants we hear occasionally), and I realized that it is just not the right choice for me. I am a person who likes to explore every option of my devices and the trade-off of super-simplicity instead of more function seemed limiting to my nature.
Long story short, I sold the (immaculately well kept) PB to somebody at school whom I hope it'll serve better. To this date, I limit myself to watching Apple's innovations and continue the (perpetual?) wait for that ultimate form-with-function devices!
