Location, location, location - quite frankly, it doesn't get any better than this. The apartment is located in what is the heart of Old Town, about two minutes on foot from Charles Bridge (two very leisurely walked minutes even - really, it's incredibly close). Despite the fact that the apartment is located in the middle of the action, we had absolutely no noise complaints whatsoever. A combination of the fact that the apartment faced a somewhat quite back alley (so all the partying happens at the other end of the building) as well as really good sound insulation made for an incredibly relaxed and quiet night of sleep.
The other thing, besides the incredible location, that really stood out for us was the friendliness and the willingness to help from the people at the reception. Whether it was assisting us in calling a taxi, lending us an umbrella for the one cloudy day we had, or simply allowing us to leave our luggage in a safe zone after check out (while we still had a few hours to do some more Prague exploring before we had to catch our flight), everyone at the reception was super friendly. Also, even though basically everyone we talked to in Prague spoke excellent English, we do feel like mentioning this again specially for the receptionists - there basically were no language barriers whatsoever at any point in time.
The actual apartment was well beyond expectations as well: very spacious, very, very clean, and super comfortable to spend some time in. You know you've got a great place to stay if you don't mind coming back to the apartment early one night because you're a bit worn out from all the exploring, and you can just relax and feel at home in the apartment - which is exactly what we did. All commodities you could wish for were there: a very spacious bathroom with a great shower, a small kitchen with everything you need to take it easy in the morning and prepare breakfast (including a coffee maker and some excellent coffee pads to go along with it), and even a small fridge with some "welcome beers" in it - a nice little touch! Finally, the apartment has quite a good heating/air conditioning system, so if you're anything like us and like to have your room nice and fresh at night before you go to sleep, you're set in this place.
To summarize: the location is perfect, the apartment is super clean, well maintained and has all the comfort you could wish for, and finally, the place is spacious enough to feel like a little home-away-from-home. Highly recommended, we would stay here again!