We had a great time staying at Guillermo's place in Madrid. The apartment is clean and nicely furnitured, very quiet (although there can be a lot of noise outside on the streets - but you hardly hear anything up there in the apartment) and it's a good tums-bums-and-thighs workout to walk up and down to the 4th floor every day. :-)
Lavapiés is a nice area with lots of bars and restaurants, nice little shops, plus you can practically walk all of Madrid from there.
Would definitely stay there again!