All in all, it was a pleasant stay. Manuel welcomed us very nicely with a small gift and showing around the whole facility (including the garage) and the neighbourhood (restaurant Hernani, great local restaurant).
Upsides:
1. Functional apartment, well located (5 min walk to the Côte de Basques, 10 min walk to the city)
2. Well equipped kitchen
3. very nice and spacious balcony (3rd floor, lot of plants)
4. If you come by car, the underground parking is of great value in Biarritz! But be advised, that it is narrow (an Audi A4 just fits in).
Downsides:
1. Be aware, there is NO Wifi.
2. The bed (as stated in several reviews) is really not comfortable. After a few days we really started to feel the bedstead through the thin mattress.
3. In the bathroom, you cannot really use the rain shower without flooding the bathroom.