This casa is WAY overrated. After spending 3 nights here last month, my wife and I are scratching our heads over the high ratings. Our experience: dirty bathrooms, toilets that don't flush, stains that looked like blood on the bedroom floor, lack of a comfortable space to hang out (the owners' living quarters are separate... and way cleaner). Yes, the owner is friendly, chatty, speaks good English, and was kind enough to meet us at the airport (though the fare cost us more than what we paid for a cab we found on our own... everyone in Cuba adds a finder's fee for every service rendered). But you can find WAY better accomms, and safer, even in gritty Old Havana.

Calle Luz is several long, dark, gritty blocks east of Plaza Vieja, in the heart of the poorest part of the city. My wife and I avoid hotels and opt for casas and B&Bs wherever we traveler, so our expectations were more than tempered in Cuba. But this place put us off immediately. We wanted to check out the minute we got there but decided to tough it out. Things just got worse: on our last night my wife got jumped by a thief who grabbed her purse and ripped her blouse, just a block from our front door. When we got back to the casa and told the owner, all he said was "hey I told you, don't walk down dark streets at night." Gee thanks. Hey, if your idea of adventure is staying in an unclean apartment in a slum and you like mildew in your shower and food from a dirty fridge, this is the place for you. If not, keep looking!

We chose this casa for its central location in Habana Vieja. It is within walking distance of the Capitolium, the main shopping streets (San Rafal for the more local shops) and Obrapia or O'Reilly for the most touristy ones, as well as the beautifully restored Plaza Vieja, Plaza las Armas and Plaza Catedral.
The street itself may seem frightening at first - we were two young women travelling together and were first shocked by the amount of attention we were getting from the local men. Once we got used to it, we walked the streets comfortably, even at night returning from a dinner on Plaza Vieja or nearby Mercadero.
The Casa owners were a friendly elderly couple whom I believe we met (most of the time there is an owner/manager there, taking care of the bookings and helping in any question you have who speaks good English). There are some other persons taking care of the food and cleaning during the day/evening, so there is always someone at home to let you in.
This casa is on the 2nd floor of the building - just to remember if you are coming with a large piece of luggage and the taxi driver won't help carry it up for you. Also, just to keep in mind - the 3 bathrooms open from the hall/foyer area and are used by all the guests.
The large dining room in the semi-opened terrace is refreshing on a hot evening and the rooftop also has a table and a hammock to relax in after a long day. The casa is in the heart of town - so if you are a light sleeper, bring your earplugs as life stops late / starts early in this part of town!
This is a busy casa with returning guests so book in advance - there is internet connection so its likely you will get a quick response.

Most of it has already been said here, I can just confirm that the atmosphere is great, the location perfect for old Havana, the staff very friendly, the food is good and the rooftop terrace amazing. Say hello to Che if you stay there.

Make sure to check for availability in advance, as the place is quite known and therefore can often be sold out.

Enjoy your stay :)