I visited Berlin for 5 days in April.
The apartment was very beautiful, and comfortable. It was also well lit, with large windows in the living room, which was very nice.
The location was good, in Neukölln neighborhood. Very close to an underground train station, easy to get to anywhere from there. It was a walking distance from Tempelhof field, which was the site I most wanted to see, so I was happy about that.
Also close to a shopping mall, that had a huge supermarket where I could buy groceries. Some interesting restaurants on the nearby main street as well (try "Paolo Pinkel").
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There was an unexpected temperature drop in the week I was there, 4-6*C during the day (the week before that, I was told, was 20*C). It was very cold outside, and the heating in the apartment didn’t work. Possibly because April is considered Spring in Germany, and the building turned the central heating off. Leah was very nice and she came by right away and tried to fix it, and provided extra blankets, and that really helped at night.
I know that for many people this won't be a big deal, but for me personally it was difficult to travel and see the sights at this weather (I come from a warm country...), and also I didn't really feel that I can enjoy the apartment in the evenings, just sit and read a book or something, because it was too cold for my taste. So if you are visiting Berlin in April, take under consideration that the weather is unpredictable, and you might not have heating. And if you do not enjoy traveling at this weather, like me, then don't visit Berlin in April :)