The first thing I think you need to know about this apartment is that it is on the fifth floor (sixth by American numbering conventions) and there is no working elevator in the building. The host forgot to mention that so I'm mentioning it for him.
You also should make sure that you insist on more than one set of keys if you have multiple people in your party, as the only way to get to the apartment is through an iron gate with a deadbolt lock that requires keys to open in either direction. Which means that when I was in Prague for some meetings and my brother came to join me he would have to get up in the morning, go down five flights of 3 with me to let me through the iron gate, then turn around and climb back up the five flights of stairs. Seriously, if you have multiple people who are on separate schedules you will need to insist on multiple sets of keys.
Those are the bad parts worth mentioning; I had some other quibbles but they are very minor in comparison. What about the good parts?
It is, in fact, a large spacious apartment, clean and bright, and very conveniently located in the heart of Prague. The kitchen is nice, there's laundry on site (including a dryer, which is relatively uncommon.) The dining area is sunny and pleasant. There are good views from the windows.
There's a tram stop about 50 meters from the door of the building, a metro stop three blocks down the street (next to the shopping mall) and it is an easy, pleasant, and safe walk to Staroměstské Náměstí, the square in the heart of Prague's old town.
There are grocery stores close by, and restaurants, bars, bakeries, historic sites all within easy walking distance. It's also nicely located on the border between the old town neighborhood and a less touristy part of town that's slightly less touristy, for when you want a meal or a drink without feeling like you're actually in a Prague-flavored theme park.
Overall we enjoyed our time in this apartment but I passed over other lodging and paid a premium for this apartment, hoping to have a special visit with my brother (who lives several thousand miles away from me ordinarily and whom I rarely see.) It *is* a pretty apartment in a good location but the access-related inconveniences killed off most of the luster for us. When our original booking expired we moved to another nearby apartment that was 2/3 the price and less pretty but much more convenient to enter and leave and we were happier.