The place is great. Really worth it. That's why I'll first write the only two possible reasons not to choose it. If these are not a problem for you and your mates, go ahead on reading or just book it because it's great.
1) If you have mobility issues having to walk up 5 floors, though not as high as modern buildings floors, might be a problem. If you don't have particular health issues it's not a big deal.
2) If you are three or four very tall people you might have a problem with the sofa. Though being very easy to open up as a bed and comfortable to sleep in, it is too small for people who are 1.80m or taller. In our case the two shortest people (who are 1.65 and 1.70) slept there with no problems, and the other regular bed was fine the other two of us (who are 1.85).

Now to the actual review.
The apartment is right in front of the Anton Martin metro stop, it was very easy to reach with public transport from the airport, no need for a taxi. The building is old but the apartment feels very modern, it is very well lit and properly insulated. The bathroom is nice, though a bit small (bigger people than us may have trouble moving around and sitting down). The apartment is just like in the pictures.
The location is really convenient if you want to visit Madrid properly. In the beginning we were thinking of getting a public transport card, but then realized how we were right in the center of all the things to see and decided to walk every day. You just pick a direction and you always get to one of the main places in Madrid within half an hour (and see a lot of interesting buildings and streets while walking).
Even though we didn't meet Lucas personally, he always replied almost instantly to messages and gave us all the information we needed.