You can't really beat the location of these apartments if you're wanting to explore Warsaw. They're located in the Nowy Swiat area of Warsaw - a hip and trendy place. They're a short distance (two tram stops + short walk) from the Central Station, as well as short walk (or bus) from Old Town.
I've stayed here twice, and enjoyed my stay each time. The apartments are very spacious, super clean, and have everything you'd want for an extended stay: washing machine, dishwasher, stove, wifi, etc.
Two minor caveats to note:
1) is that things can get a little noisy at night if you have an apartment that has windows opening onto the Nowy Swiat street (drunk people stumbling), but that's the price you pay for being close to some bars ;)
2) make sure you understand the locking mechanism on the door - there is the possibility of locking someone inside if you turn the key twice, however I think this was only possible on one of the locks...