This is really awesome; except as an app developer, I really rather get a library that I own and have on my own infrastructure than a SaaS model, especially for something so critical to my application.
If you just want the raw "back end WebSocket-compatible multiplexing daemon" functionality, that's not hard to code/devise. You can use something like: http://github.com/gimite/web-socket-ruby - What services like Pusher provide is all the API stuff on the front, maintaining the servers, keeping those daemons running, and, well, just making it super easy to get going without worrying about the back end so much.
All the API stuff on the front I'd pay for. However, I'm already paying for "maintaining servers, keeping them running, and worrying about the backend" -- so I don't want to pay for it again.
This might just be MY preference, and others might beg to differ.
It depends on the type of app you're building. We built Pusher to complement existing AJAX workflows and make adding collaboration easy. There's lots more you can do with it though.
We tried nginx http_push_module before building pusher, and it was definitely one of the inspirations. I'd recommend it if you want to use long polling and are happy to maintain your own infrastructure.