I recently stayed in Kate's flat with some friends for the festival, and I have nothing but praise for the experience! Kate was a splendid host, confirming details of our arrival in advance and always quick to reply to any questions we had. Her flat was everything we had hoped for--sparkling clean, the perfect location for exploring Edinburgh, and plenty of space to accommodate a group of people comfortably. The views of the Edinburgh Castle were another added delight.
The one thing that was less than ideal was the nighttime noise on Rose Street--but that was due to fringe festival rowdiness, and probably wouldn't be a problem at other times of the year.
Overall, I would definitely stay at Kate's again and highly recommend!