We stayed at Piwna Orange apartment - just half a block from the Royal Castle and a block or so from Old Town square. Great choice. The only somewhat problematic thing is parking (if you have a car), but usually one can find a spot on Podwale, about a block away.
Apartment was 3 rooms and an open kitchen. All nicely equipped including washer and a dishwasher. Beautiful modern bathroom, nice and comfortable (although somewhat spartan) bedrooms. Cable TV, coffee maker, microwave - all the basics. One warning - the neighbors are a bit nosy, but just don't pay any attention to them.
Location is perfect - can't dream of a better one. And the price was great, too. For the 4 of us to rent 2 separate hotel rooms would be much more and without the amenities we had there. It's not a hotel, so no room service or pool, but you feel like you own the place for a few days.
Would definitely recommend you look at renting this place if you're staying a few days in Warsaw. We'll be back there for sure.

We stayed at apartment M15 (Ul. Chmielna) for 7 days. On arrival we found a large stain on the sofa that look like semen. On the second day we noticed dog poo on the steps near the entrance. This had not been cleaned by the time we left.
The main bed was very unconfortable - the worst we ever experienced. After a few nights we decided that it is much more confortable to sleep on the sofa.
On our arrival the telephone was not working and it took several day to fix it. The toilet seat was not tightened to the base properly and the tap in the kitchen sink was loose - not properly attached to the sink.

We stayed in apartment M18 (Ul. Chmielna) for 4 days. The apartment was very comfortable and clean and the service was very good too. The location is in the town centre and it's close to everything. I recommend this apartment.

We stayed at Appartment M21 and it was great. We had walking distance to everything, Malls, clubs, restaurants, museums and "the old town".

The appartment was excellent for 2 people, with a nice kitchen and very spacy livingroom. The bathroom was a bit small, but not so much that it was a big problem.