From the view of the Castle when you get through to the courtyard to everything in and around it, the Wee Highland Coo is perfect for a short stay in Edinburgh. There is plenty of space to relax inside the apartment and plenty of good bars/restaurants nearby. Getting to the castle and the Royal Mile/the rest of Old Town is also quite easy, but be prepared for stairs through The South and North Wynds for a shorter route closer to the entrance.
Highly recommend having Gavin as your accommodations host, as he is very responsive anytime you have questions, etc.. He's included a book which includes local recommendations and instructions on how to operate anything you're unfamiliar with in the apartment. Would definitely stay at one of his properties again if given the chance.
The only downside, which is not a fault of the host by any means, is the street noise after bars start closing. It wasn't that big of a deal to us, but it does fade away around 1:30-2.
Thanks for a great stay, Gavin!