We stayed for four nights at the beginning of our trip, and another night at the end, and can't complain about a thing. The Catedral is surprisingly cheap (with a 10% discount for paying in cash), extremely convenient (a minute or two's walk from the Cathedral, Templo Mayor, and National Palace, among other Centro Historico highlights) and friendly. We had a fifth-floor room facing Donceles (away from the Cathedral), with a balcony and a great view of the charming cobblestoned street, and then a second-floor room, and while the higher room was better, they were both nice enough, especially for less than US $50. The second floor room didn't have purified water on tap, but bottled water was provided. Our room on the fifth floor at the back wasn't loud.
While (as other travellers have noted) the Centro Historico is pretty deserted at night (although we happened upon a fun, if smoky little 80s-gothic-industrial club night a block and a half away, on the top floor of a computer school, one Friday night, if you're interested in dancing) we never felt unsafe walking around, even quite late. We also heard from hotel employees, cab drivers, and others that the last mayor of Mexico City has made great improvements to public safety. But, as with other hotels, the staff is happy to call you a car if you're feeling uneasy. They use the most expensive, unmarked 'turismo' cabs, but they're very convenient, and by US city standards the fares aren't unreasonable -- around $100 (US $10) for a trip across town to Condesa, for example, or $140 (US $14) to the airport.
The restaurant offers the best breakfast in the neighborhood -- the food is even better, in our experience, than the highly touted Casa de Las Sirenas around the corner, although without the same spectacular view.
There are at least three Metro stations within easy walking distance, so you can get anywhere in town for a paltry 2 pesos (US $0.20). In terms of cleanliness, ease of use, and frequency, the subway's one of the best I've ever used, although it can get crowded.
I can think of only two big reasons to avoid this hotel: if you want to be within walking distance to nightlife (try Condesa for that, or the Zona Rosa), or don't like crowds. Although abandoned at night, the neighborhood is EXTREMELY bustling during the day, when Calle Donceles and the other streets just off the Zocalo become a colorful, chaotic, jam-packed street market. I loved it, but cab drivers and the claustrophobic aren't so fond.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.