PictoChat is good for when you're in a big crowd, but other than that, it's pretty useless.
When you load up PictoChat, you are given 4 rooms (with a maximum 16 people in each room) to choose from. You use the touch screen as your virtual paper, and you can draw whatever you want, or hit keys on the virtual touch screen keyboard to type a message. The bottom screen is your "paper," and the top screen displays the chat room, connection strength, and users in the room.
The only real use for it is when you're in a crowd. For example, at E3 06, everyone supposedly had their DSes out, PictoChatting away. Personally, i'll be using it at the Nintendo Fusion Tour, where there'll be lots of people. That's the good thing about this built-in feature.
Overall, PictoChat is a good idea marred by the inability to go online via Nintendo Wi-Fi Connection.