I tried using faye and it died very quickly under load with the redis backend. For every message that you send it queues it up into a redis list for each faye server in the cluster to read which doesn't scale very well in their very chatty redis queue system.
I would do significant load testing for each use case before using it (or socket.io for that matter). I needed to push 30-50 messages per second to each connected client and faye started choking as soon as there were more than 20-30 clients. socket.io would choke at around 50-100 connected clients. Raw websockets were able to reach closer to 100 clients but performed more or less similar to socket.io.
I would do significant load testing for each use case before using it (or socket.io for that matter). I needed to push 30-50 messages per second to each connected client and faye started choking as soon as there were more than 20-30 clients. socket.io would choke at around 50-100 connected clients. Raw websockets were able to reach closer to 100 clients but performed more or less similar to socket.io.