The apartment is beautiful, and perfectly located. The bed is comfortable, the kitchen has everything you'll need, and the living area is spacious. It's up only one very easy flight of stairs, so even with luggage you shouldn't have any issues. There's a grocery store (Sainsbury's) just a couple blocks up the road, and a fantastic place (Mary's Milk Bar) for coffee or hot chocolate just downstairs. It's also wonderful to have a small convenience store just under the apartment. Easy walk to the Royal Mile - less than 10 minutes away. It's also pretty quiet, even late on weekends when the Grassmarket is hopping.
For a cheap dinner, take a 10 minute walk over to The Piemaker on South Bridge - try the Vintage Cottage pie for (I think) under £2. If you don't like pie, there are many other dining options around. The team has also done a great job putting together a book of local information - it really has everything you need to know.
This is a fantastic apartment, and you won't regret staying here!