I stayed here in September 2016. The apartment is in an excellent location for sightseeing, Karakowskie Przedmiescie and Ul Marszalkowska being only a 5 minute walk away in opposite directions. It's a fairly safe area, though like any city, just be aware of your surroundings. As the pictures testify, it's very modern and clean inside, and has everything a group of up to 6 people need. The owners (Anna and Piotr) are very pleasant, and we had no reason to need their help during our stay, but reviewers elsewhere have said they are very quick to respond to any issues raised.
The apartment is located on the 5th floor and there is a lift, so no need to climb the stairs unless you need the exercise!
The bathroom is a good size with a big shower, and towels and soap are provided. Plates and cutlery are available, with a fridge, cooker and microwave, so self-catering is not a problem here.
The bedrooms, although very small, are absolutely fine if you're only using the apartment...