Having stayed at quite a few casas particulares throughout Cuba, I was VERY pleasantly suprised by La Casona de Obrapia... This is not "a spare room in someone's house" as per most casas, but rather an old building in Havana Vieja specifically renovated to provide basic accommodation for tourists. As such, it feels much more like a high-end hostel, or a budget hotel, than a typical casa (so if you're looking for the "living with a Cuban family" experience, you won't get that here; but if you just want somewhere clean & simple & inexpensive to stay, with a more hotel-like level of privacy, this is the place for you!). Since everything is new, the place feels relatively modern and is spotlessly clean. Yes, the bathrooms are tiny - but they are brand new and that counts for a lot in Cuba! The rooms are very basic but provide everything you need - a/c, fans, blankets, crisp white sheets (a luxury in cuba!) and comfortable beds. And pillows that don't feel like lumps of mushed-up foam (you'll know what I mean if you've spent any time at all in Cuba!). The ceilings are high, and the common area with sofas is a nice touch. Only real downside is no windows/natural light in rooms - but this is pretty common in Havana casas I think.
The owner, Raydel, is very helpful and speaks decent English. He can also help you out with finding accommodation in other towns (eg. Vinales) if you're moving on from Havana...
For 30ish CUCs per night, I think this is a great value accommodation proposition you'll in Havana. Excellent location in the middle of Havana Vieja - street feels a bit gritty, but you're just one block away from all the cafes & restaurants of Calle Obispo.