Folks visiting Berlin cannot go wrong with Heike's place.
Let me first start by explaining that Berlin is a massive city. You cannot be 'walking distance' to everything, so that means you want to be central in terms of public transit. This place is a 3 minute walk to the Frankfurter Tor U-Bahn (the metro). It's a few stops away from a hub U-Bahn station (Alexanderplatz), from which you can go anywhere. So as far as ease of transportation, this is perfect.
In terms of the neighbourhood, Friedrichshain is one of the hottest new spots. From the flat there are a couple nearby streets filled with cool bars, restaurants, coffee spots (Silo!), two delicious Indian restaurants (Chai Ji and Seth) and plenty of other interesting places including a market.
The spot itself is super quiet, so we had no issues with any outside noise disturbing our sleep. The flat and building is super interesting, especially if you're interested in what East Berlin was like. It's comfortable, has great furnishings, and the building itself carries the East Berlin look and feel. There is an elevator too, so you don't have to walk up the flights.
We enjoyed the week we stayed there and would stay there again if we visited. You can't beat the convenience & location.