I had the pleasure of staying with my girlfriend at Charles' place for three amazing weeks. The location of the apartment is ideal, specially if you have a car -free parking only 5 minutes away-. The apartment itself is nice and modern. The city centre and the main beaches in Biarritz are within walking distance. Checking in and out posed no problem and Charles' father was of great help providing us with maps and valuable information about the city.
To sum up, I would totally recommend staying there, I would certainly be coming back!