addFilterObservableToWeb3.js

"use strict";

const Rx = require('rx');

module.exports = function addFilterObservableToWeb3(web3) {

    web3.eth.filterObservable = function(_options) {
        const filter = web3.eth.filter(_options);

        return Rx.Observable.create(function(observer) {
            filter.watch(function(err, result) {
                if (!err) {
                    observer.onNext(result);
                } else {
                    observer.onError(err);
                }
            });

            return function() {
                try {
                    filter.stopWatching(() => { });
                } catch (error) {
                    console.log("Geth probably disappeared early", error);
                }
            };
        });
    };
};

And you use it thus:

const Web3 = require("web3");
const web3 = new Web3(...);
const addFilterObservableToWeb3 = require("./addFilterObservableToWeb3.js");
addFilterObservableToWeb3(web3);

web3.eth.filterObservable(myContractInstance.LogThatEvent())
    .subscribe(anEvent => {
        console.log(anEvent);
    });