Me and my wife stayed at Rainer's Place three nights at the end of August. Rainer is a very friendly guy and made us feel very welcome. The flat is very well located being almost at the city-center and very close to a subway station (Schwedenplatz). However this flat is not for everyone. If you are not willing to climb the 84 steps up to Rainer's flat or you are expecting quiet neighborhood at night, this will not be a pleasant stay. We were determined to have a good time in Vienna and we didn't let those things get to us. We had a decent sleep after we put in our earplugs.
Bottom line is that the flat is fine, the host is great, geographical location is great. But all this comes with a drawback that future guests should be aware of, especially light sleepers.