I can't say enough good things about Apartment 21. It's located in Mitte (literally means center) neighborhood of Berlin, making access to the entire city either walkable or bikeable. There are beautiful coffee shops, craft pubs, and delicious places to eat within a 5-10 minute walk too.
Carolin is one of the best hosts to communicate with prior to and during your trip to Berlin. She provided specific directions from the airport, along with tips for anything we needed.
The apartment is beautifully furnished with everything you need, including wonderful and interesting pieces of art and all the European contemporary design touches. Beds are comfortable enough, especially if you're a stomach sleeper. Linens are clean and appropriate for the amount of guests. Carolin also provided an array of fruit and bag of pretzels too.
Overall, we couldn't have asked for a better stay in Berlin.