Both Raphaela and her neighbor (who handled the keys) were friendly and responsive to all communications from us both before and during our trip, and we were very satisfied with our stay. The apartment is indeed large and bright (the pictures and description are accurate), and it was very clean for our arrival. It's not fancy, and it's not a tourist apartment (they live there, so don't expect empty wardrobe or drawer space), but it is quite comfortable. It was surprisingly quiet for the location: with the windows closed (this was December), we heard very little noise from outside, and we never heard much from inside the building, either. The heating was great (too hot at first, in fact, but we were able to turn the radiators off), the mattress was comfortable, and the shower was impressive (hot and powerful). The washing machine was easy to use, which was much appreciated for a longer stay (nearly two weeks, in our case). The kitchen is a bit small but well organized and well supplied.
The Stasi Museum is within easy walking distance, there are decent döners on the block, and the location is very convenient for getting anywhere else in the city (airports, train stations, tourist sites, friends in other districts), with bus and tram stops right outside the door and a large S- and U-Bahn station (Lichtenberg) just across the street.
Complaints? None of any significance, really. The main light fixture in the kitchen wasn't hooked up, and the one in the bedroom required a fine touch, but those things seem to be the last remnants of an otherwise thorough and successful renovation and will surely be cleared up before they host again. Note that there is no real dining or breakfast table, so you'll have to eat your meals on the couch at the coffee table, so that's a bit odd, but not a tragedy.
All in all, great value for a great stay, and we'll be happy to stay there again if the dates work out for our next visit.