I'm running the Arduino board continuously to control a hot water solar panel pump based on the data from several temperature sensors. The board itself is very reliable, but occasionally the serial port hangs and needs to be disconnected and then reconnected - though this might not be the Arduino's fault. I have, however, seen this behavior when using it with both Linux and Windows boxes. But I'm connecting through a cheap Chinese 150 foot USB cable that I know to be unreliable in electrically noisy environments.