In the post I have seen, where the guys got a single full rack and played with it on the weekends, running it for a day added a significant amount to their bills, so yes, newer systems are more efficient (generally due to compute efficiencies), but disks are disks. Spindles are not way more efficient than before.

On the ROI part, this is a case by case issue. I for one can do the "play" part at work, too. Also, I don't want to spare space for a 1U or 2U full-depth server at home. I'm not even adding disk boxes to this. I neither have the space, nor the desire.

Good for you.