A short break in DC (late review)
We stayed at the Palomar in late April, for a 5-day break, bought as a package with British Airways via their website.
The location of the hotel is not central to DC; however, it is located within 5 minutes walk of Columbus Circle on the Metro. The hotel is comfortable, modern, stylish, everything you would expect based on it's rating on Tripadvisor. The rooms are adequate size, on the assumption you aren't going to be spending much time in them. Nicely equipped, with quiet air-con, big TV (shame about the content), bigger bed, beautifully finished bathroom. Ours had a large walk-in closet, with a safe (no charge to use), ironing board and iron, mini-bar/fridge, robes and even umbrellas. Sign up at the Kimpton hotel webpage for free wi-fi. Staff were very pleasant, helpful and (more importantly) approachable and open - very different from a previous trip to NY. There is also a swimming pool, although we never found or used it.
Breakfast was included in our package, served separately from the main restaurant (Urbana) and was simple but plentiful and very good quality - enough to keep you going for most of the day. It was also very good value compared to what it would have cost to buy in the restaurant had it not been included. We did eat in the restaurant one evening, it was nice, but slightly expensive compared with the many places around the Columbus Circle area. Wine o'clock with nibbles was held every day between 5-6PM in the hotel foyer and was always popular, assuming you got back in time.
The hotel location is about equally spaced between Georgetown and the White House, both being walkable distance if you want (we walked to both). However, the Metro is so simple and easy to use (compared to NY) and the tickets work on the buses as well (and can be topped up as required).
I could only find two faults with the hotel. One, our room on the second floor, looked out onto an internal courtyard at ground level, so we had cars parked outside the window. But it's a city hotel, so that's not unusual. Two, the lifts seemed incredibly slow. But as we were only one flight up from reception, a handy service stairwell solved that issue. 
When we booked this we were pleasantly surprised to find it rated as the second highest in DC. I notice it has dropped to third since, but I can only assume that this is due to other hotels catching up rather than it's own standards slipping. Highly recommended.
A general travel note. If flying in to Dulles, a taxi will cost you 60-70 dollars to get anywhere into the city proper. If you use the Washington Flyer coach it costs 10 dollars to get to West Falls Metro, then about 2.5 dollars to Columbus Circle, and takes about the same time. Incidentally, the Metro is currently being extended out to include Dulles. Not sure when it is due to complete, but it will make it even easier and cheaper to get in.