I have stayed here several times in the last 12 months, and each time I have found my room to be spotless. The bathrooms are clean, no mould spots, the beds are comfortable and the bedding also is spotless.
Sure, it is a very old building (there is a plaque on the outside wall which says that someone of note died there in the 1850s) and it retains a lot of its original character.
Staff are friendly and helpful, and the reception desk is manned 24 hours per day.
The hotel is located in the Historic Centre of Mexico City, about a 10 minute walk from the zocalo. Several pedestrians only streets make this a colourful and interesting exercise. I have not felt uncomfortable or uneasy walking round this area in the evening.
The restaurant does great breakfasts, also lunch and dinners.
If you are looking for the Hilton, this hotel is not for you, but if you are looking for a basic but clean, comfortable hotel, Hotel Isabel should be right for you.