RPis are slow for crypto, but super compact and simple - ideal for cold storage operations IMO.
The latest model is not even particularly slow, it signs a transaction within a second or two. The initial RPi required a bit more patience ...
For the Pi3? I was tempted to try it, but the Pi3 includes onboard Wifi and Bluetooth, so I thought the Pi2 would be better for cold storage (if not frustratingly slow). I imagine the original Pi would be nearly unusable.