The ability to have an easy way to connect my server with different clients (mobile, web app). I like that it is very well documented and has examples of how to use it. It has made me more productive because when I need to write new code for something new, I don't have to search through documentation trying to find how to do it. Socket.IO is easy to get started with so you can go right away. There wasn't anything I disliked about socket.io, but there were things I would have liked if I had time in the future. You should take advantage of the free version first since it will give you a good idea of what you like/dislike and how it works. My company uses socket.io to communicate with our customers and it works great! We are able to send messages from our servers to our clients.
ο»Ώ