This is the second time I've used the Rewari as my base for exploring Berlin, both times staying there for close to a week. The hotel staff are always friendly and there's never any language problems; their daily breakfast is a good variety of breads, meats, veggies/fruits, cereals, coffee and juices. The rooms themselves are small by American standards but perfectly adequate in space and accommodations. One key factor is that the other guests are always courteous, I've never had problems of noise from other rooms and people are cordial in the breakfast room.
One key factor for me in using the Rewari, besides the overall pleasantness of staying there, is the location. It's in a quietly residential section of the Kreuzberg neighborhood, so it's comfortable leaving a window open for ventilation. There's good access to transit -- the bus to Potsdam Platz and on to the main train station stops out front and there's 2 u-bahn and one s-bahn stations within a 5 minute walk. The neighborhood feels perfectly safe, so I've no hesitancy about walking from one of the stations late in the evening.