First of all, the apartment is very spacious, much more so than what it looks like from the pictures! The location is really in the middle of Friedrichshain, which is lovely, but I should be clear that it is extremely noisy and it was almost impossible to sleep without ear plugs (there's also a tram that runs right outside, day and night). The curtains don't black out a thing - most of them are pretty flimsy sheer curtains - so pack an eye mask. I suppose the noise wouldn't be as much a problem if you slept with the windows closed - it was impossible to do so with Berlin seeing temperatures up to 35 degrees Celsius, though.
The kitchen is nicely equipped, the coffee machine capsules can be easily found in supermarkets. We would have appreciated a bit more info regarding the recycling, but I sorted that out with the help of a German friend.
Cleanliness was okay - bed linens and towels were pristine, in intact laundry bags. However, there was a lot of dust all over the place (except for the bathroom), and the TV remote was really disgusting. Would have appreciated some hand soap and toiletries, but the soap dispenser was empty.
Now, for communications - Marius took a long while to get back to my questions, and then again, I often had to repeat the same question over and over, and still he didn't provide particularly helpful info. I had a better experience with the person responsible for cleaning - Marco - who dropped the key off for me at a local safe box.
Overall, I would say we had a positive experience, since the apartment is exceedingly well located and bright - but we got little, fitful sleep over the week we stayed.