Overall, great flat right at Leicester Square - you're literally in the middle of the action! I was in town mostly for work and meetings. The flat was close to a few subway stops so I was easily able to get around. FYI, you don't need to buy a subway pass, you can simply tap your credit card as you go in and out of the stations.
The flat was spacious with two bedrooms and each had it's own bathroom. Everything was completely clean. Beds were very comfortable. Whole Foods is about a 5 minute walk away incase you want to stock the fridge (which we did). We had a wifi issue which made things a little stressful during a work day, but the host took care of it. Still, have a backup plan just in case - unfortunately my US-based mobile service was not giving me enough bandwidth so I had to go to local coffee shops for wifi. In any case, it was handled and then all was good!
The elevator was turned off in the building - I was prepared since other reviewers said the same. Just be mindful of this in case you have heavy suitcases or struggle with going up several flights of stairs. I didn't mind the exercise, but if I had brought my mum on the trip, we would have definitely had a problem.
Lastly, even though you're in the center of all the action, it's still a very quiet apartment because of its location relative to the street.