In one word: perfect.
Perfect hosts (friendly attitude, helpful info, great tips), perfect apartment (clean, warm, cosy, beautiful), perfect location (Straßen Bahn direct to 1st quarter or to U-Bahn, super market on the corner), perfect neighbors (nice, helpful, quiet).
We only regret that we didn't have a chance to meet our hosts. But we probably will, as we certainly intend to come again.