We've stayed at Alan and Kasia's place for 4 days, and it was all we ever wanted from a studio. It was close to the city center, just on the Berlin ring U-Bahn, so you could get anywhere relatively quickly. Apart from that, they've provided us with guidelines on how to get along in the city, trying to avoid the so-called "tourist traps", which is very helpful if you're trying both the touristy and non-touristy attractions of the city.
Would definitely stay there again.