Yeah it is. Net send can take a lot of network traffic resources. I have few friends in previous classes who have played around with it and pissed off the IT staff. You should use the name of the PC or the IP address of the computer you want to communicate with, so that it won't go to every host on the network.
While this command is fun to play around with, don't get used to it. MS also realized the pain it caused for ITers and removed it in Vista.