We stayed 4 nights at Mikel's place in February and actually had a blast in Bilbao!
The flat is comfortable for 4 persons and you have everything you need. The flat was clean and we even got offered a parking spot in the garage, which was really convenient.
I especially recommend exploring the area around the flat since although it seems rather 'shady' at first sight, you can find lovely spots, restaurants and local products.
Location-wise I can say that we never needed to take a bus or subway. The city center is about 15-20min by foot, the old part of the city with lovely bars and small passages is 10min, the famous market is 5min away.
All in all this is not the luxury choice but has incredible value for money!
I'd definitely recommend and come again.