Had a good experience in general.
The studio itself is neat and small indeed, but the area it is in is neglected; there are nicer places in the city. Everything in the studio was very clean and tidy. Kate provided us with some basic groceries that made it easier for us to snack something in the mornings or during the day.
There are some tiny markets in the vicinity with a basic selection of items that was just enough for the 3 days we were spending in Lisbon. If you're planning for a longer stay, however, this might be inconvenient for you.
Regarding transportation, getting to the Metro station will take you ~15 min on foot through a *very* hilly area (mind that!). On the way to the metro everything is fine, since it is downhill. However, the way back might prove challenging when the sun is shining at its brightest :).
Could not use Wi-Fi for the length of my stay, the neighbor that was supposed to share the key with us didn't turn it on those days. In the end, since we are EU citizens we could roam like at home, so it wasn't such a problem for us. However, for non-EU guests this might be an issue; I would advise to check on this particular item before your stay.
Hope you enjoy Lisbon!