Some computer experts say it is the best thing rather than restarting. I tried it but there was still loads of heat coming off it so it must cost something to generate that.
A secondary problem was using a KVM switch (This runs two computer from one keyboard, one mouse and one monitor switching between both). When I came out of standby they would not work until re-boot so it seemed a bit daft to leave it on.
I do not know much about computers or computing I always leave mine on standby, and other than clicking on Speedtouch and wanadoo/orange at restart - no probs.