I'm a PhD student and stayed at Gonzalo's whilst I attended a 4 day conference. It's firstly great value for money, and you get an excellent host who goes out of his way to ensure you are doing ok once you arrive. The single room is a bit tight and next to two shared bathrooms, which can get a little noisy, but you can speak to the host about other options for larger rooms if you wish (again, given the value for money this wasn't a problem for me). My belongings were safe and there is a sturdy lock for leaving them behind, so no worries on that front. Amazing location - 20 mins away from the very centre of Madrid and even otherwise, Madrid had the best and most dense Metro system I've seen yet. In short, if you are on a budget and want to stay where the buzz is, stay at Gonzalo's!