I have a couple of dozen Raspberry Pies. I dont know exactly how many, because I use them for all sorts of odd jobs. Three of them are driving my displays of time-temperature-alertcount-gardenview. One drives the robotic arm, one controls the "night mail" cache puzzle, several are name servers, mail servers and suchlike. They cost £16, and for that, you get a full-size computer (just add keyboard, mouse and screen) in a credit-card sized package.
One of the reasons I like them, is that the draw just a tiny amount of power. A full-size computer pulls 300-400 milliamps, that's 75-100 watts. The Pi draws less than 3 watts. And now there's the Pi 2, with twice the memory and six times the processing power (I suspect that in actual use, I won't get six times). And it pulls 2 watts. They cost 50% more, but that's only £25 compared with £16.
The power draw is important, because I pay a lot for electricity; each kilowatt consumed in the Data Shed, has to be extracted via the air conditioning, which doubles (or more) the power draw, depending on the outside temperature (in winter, the aircon does very little).
I power them from an ordinary PC power supply, fed through a small device that reduces the voltage to a steady 5.6v, and that connects to a USB hub, and several Pis can be powered from that hub. I don't bother with a keyboard or screen; each Pi is connected via an ethernet cable into my network.
So in my latest project, I've been pressing into service every Pi that I can find that's not doing anything. That only gives me a handful, so I decided to get more.
I've ordered five Pi 2 Model Bs to add to the stable.