We had a great time at the Katharina apt. The friendly and helpful Jaroslav met us at the apt. upon our arrival (which was a bit earlier than the official check-in time) to let us in and provided us with helpful tips. The location is perfect - just across the Charles bridge from the old town, below the castle. So it is not as crowded yet very close to the main touristic areas. The apartment is spacious for a couple and is very reasonably equipped, including coffee and tea for the guests. Lenka and Jaroslav were very accessible and even arranged for airport transportation at a very reasonable price.
One note - the room windows are just above a small nice cafe that is open from 11am-10pm . The cafe guests chatter can be heard in the apt during the afternoon. Not a major problem though, just something to know.
One of the best stays we had with airbnb for sure.