It uses a RaspberryPi internally, but you can treat it as a sealed appliance to be honest (I've had to pull the SD card once to reflash for a big upgrade). The only fiddly bit is the setup for the processing of the inputs into data streams. It's all web-gui click button, but because it's so flexible it's more complex than other boxes. The guide does a really good job of walking you through it, and if you've got a programmer mind-set it clicks once you've been walked through. Then you can add your own customisations if you want them.The emonPi is quite interesting. I'd wondered about getting a RaspberryPi to help start teaching my children to programme. I suspect tho I'd maybe not have the time to fully make use of it at the moment.
It's not for everyone though. Others put a more polished front-end on.
I initially bought it a couple of years back to get a picture of how I was using energy, as smart meters were provider-locked at the time. I reckon it's paid for itself in that time, and now with the demand shaper module (which came out last year) it's become key to saving me more money. The guys at OpenEnergyMonitor are really down to earth and happy to help anyone who's trying to make the most out of their energy use. They seem to be driving a little local Welsh industry in energy smarts. Hence I'm always happy to recommend them.