Absolutely flawless experience. The flat is very spacious—lots of room in the living/kitchen area, and high ceilings make it feel even bigger. It's on a main street, but set off in a courtyard, so no problems with street noise. The location was fantastic; plenty of gorgeous neighborhoods to explore within walking distance, but within a block of tram and U-Bahn stops, so getting to the rest of the city is also really easy.
(Side note: if you are vegan, THIS IS YOUR FLAT. There's a Denn's Biomarkt at one corner with plenty of non-dairy milk options, health foods, etc, and at the other corner is a vegan bar and coffee shop. ChaosTheorie, The Lucky Leek, and Veganz are all within walking distance, as are about ten or twelve other vegan restaurants. I was in heaven.)
Carolin was very communicative, friendly, and super helpful with information about getting the keys, local shops, etc. (I didn't realize I was arriving the Saturday night before a bank holiday Monday, so her heads-up that I'd want to do any grocery shopping on the Sunday, complete with directions to one of the few Berlin grocery stores that's open on Sunday, was really useful.)
Overall, just exactly what you hope for from an Airbnb experience. Thank you so much, Carolin, if (when!) I return to Berlin I will definitely check on this flat first!