Great location, you can walk pretty much everywhere in Bilbao from here and loads of great bars & restaurants nearby (+ Guggenheim!). The flat itself is very clean and nicely furnished. However, the sound isolation is really poor so bring earplugs - unless you want to be woken up every time the neighbour flushes the toilet!
There’s not much natural light inside and it’s got a bit of a damp/humid feel, but overall it’s a good spot to stay in for Bilbao. And the host is really lovely and cool, so that’s a big bonus!