"AJAX is usually faster than Socket.IO to “code”..."
I tried node.js and some modules a couple of weeks ago. It was pretty easy to set up and I had a "working" chat application in few hours with socket.io. It was 20-30 lines of code, provided callback to send data immediately to other clients.
I don't think polling the server every 500 ms is a good practice, say 5-10 sec or more can be reasonable, but with socket.io you got it instant.