Apex Waterloo is a fantastically located hotel as so many things Edinburgh has to offer is within walking distance: Edinburgh Castle, Royal Mile, Holyrood, and more. The parking garage is right around the corner and there is a large indoor shopping mall in the same vicinity.
Our room was comfortable enough for three (two adults and one 11-year old), with my daughter sleeping on a pullout couch. The shower was large and the water warm.
The only thing about the hotel we didn't prefer was the dining experience. The staff was incredibly nice and attentive, so no issues there, but we weren't fans of the food itself. It may be how scrambled eggs are prepared in Scotland, but they aren't what we're used to as we cook ours longer. But, I feel food experiences are a bit more personal than other aspects of travel so I'm aware others will have differing opinions.
Overall, I feel this hotel is a great option when staying in Edinburgh and would recommend this to others in the future. My wife and I have already discussed staying there again when we return. Enjoy your stay!