We really enjoyed our stay in Grünerløkka. The appartment was cute and had everything we needed. Getting the key and finding the appartment was slightly confusing, as we didn't realise we had to pick it up in the 7/11 shop and the doors weren't labelled. We'd suggest asking for specific information beforehand.
Apart from that, we felt comfortable in the appartment and it's the perfect spot to explore Oslo. It's directly in Grünerløkka with cute cafés and dinner spots. It's a 20 minute walk to Oslo central station and other attractions like the National Museum, so perfect for people who don't have a car :)