I built one. And while it worked, it was never as readable or maintainable as if we had skipped Rx all together and written using other parallel paradigms.
But we kept on running into subtle issues because when we wrote the code we didn't fully grok scheduling/threading. It took us 3 months of spending 30%-50% of our time writing reactive extensions code to finally grok it, and we were highly motivated to learn. We eventually realized there was just no way the next developer who came on board could troubleshoot or debug it. So we ripped out all the reactive extension code and our app became a little less performant but a heck of lot more readable and maintainable.