It's especially useful if you need the concept of room. They have a multi-room chat sample which gives a good start. For example, this would be quite useful if you wanted to build a MMORPG game with different maps/rooms where each user is in a single room at a time.
I'm using it with the concept of rooms and instead of sending chat messages to users connected to a room, and send JSON messages of different types and based on the type of message received by the client, I use backbone.js to update the UI based on what's received. When a user's action affect other users' views, message a dispatched back to the room.
The only problem I see with nowjs is that if you need horizontal scaling, you would need a license for a more scalable version of nowjs that doesn't seem to be ready yet from their website. Since I'm thinking forward for my pet project, I wouldn't want to pay for this. I'm thinking to add a Redis pub sub layer to take care of horizontal scaling but would love to have access to their solution for this!