We stayed for 2 nights in this apartment hotel. We had our 2 year old with us, and this was a fantastic stay for a few reasons.
1. The team is highly communicative and amenable to requests. Originally they had given us a baby cot, but my toddler has outgrown it - so they were gracious enough to give us an extra bed, with no extra charge. They accommodated an early check-in to make us comfortable.
2. The room while small, was exceptionally well designed. Even with a pull-out bed, a sofa, a small dining table - we never felt cramped. The furnishings were modern and high quality, and having a small kitchenette in the room was helpful.
3. The location is amazing - a short walk from the Royal Mile, and on a street that was quiet at night. They have rooms that face the castle - though they were sold out of those during our stay.
4. Finally, it's the small things that make a good stay great. They left a super cute soft-toy for my toddler on our departure date, which he absolutely loved.
One small downside - you have to take a staircase with about 20 steps to get to the rooms. There are no elevators to carry your suitcases. This wasn't a big issue for us, but if you are mobility impaired, it could be a challenge.