We didn't see much of a need to expand beyond long polling, if for nothing more than simplicity's sake. Realtime is a small feature in small parts of the application right now. Keeping it very simple and coupled with our RESTful API let us roll it out quickly.
Main reason to avoid long polling is the awful scalability (compared to plain old socket programming).
Processing the poll request certainly has a bit of a CPU/memory/network overhead. Though any sort of keep-alive would require something similar. I'm sure we'll get fancier with WebSockets when we have to rent too many new servers because of this stuff. The simplicity and elegance of overloading the RESTful HTTP system was the most appealing bit for me with this architectural experiment.
As I understand, Server Sent Events are plain old HTTP, unlike Web Sockets.