Used in a number of real projects. API is very good and straight forward, dev team support also great.
I was able to employ web sockets on one project which really made it fly. Real-time financial price distribution.
The server side is wired up using Guice, which we struggled to sensibly(ended up statically accessing stuff) make work with Spring, so could do with a bridge there.
I wouldn't necessarily recommend the GWT RPC substitute. While it works, it synthesis's subscription requests under the hood, so one simple call results in a number of http requests. (subscribing and unsubscribing) to a GWT RPC single call. Played havoc with ntlm and IE, re-authenticating every post requests. Firebug/Chrome dev tools very handy to see what's going on.