I'm trying to find some actual latency benchmarks for using WebSockets in HTML5 games. If I were to go down this route, I'd probably end up using (as you might expect) Node.js and Socket.IO.
However, most search results I find tend to discuss what is possible or how it would be done. I'm more concerned about what kind of performance you can get out of this platform.
These are the only demos that I've found that include a latency meter for the user:
Most of the time it stays around 100-200ms.. Not sure how reasonable that number is for other games out there, although I have come across people using long-polling on top of Node.js, which gets you some pretty bad lag..