I really enjoyed staying at Casa RM26. A lovely, cozy and nicely designed place, with a perfect location in Roma Norte. Alejandro is an incredibly attentive and helpful host. Rooms have everything you need, and there is a cool common area to meet other travelers, in this hipster hostel-type spot.
It was quite cold in CDMX during my visit (winter), but Alejandro saw to it that I got an extra blanket and even a portable heater, right away. I never actually met Alejandro in person, but he answered messages right away, and helped in every way possible.
Only potentially negative thing about Casa RM26 is that there is very little sound proofing between rooms, so sound - conversation and otherwise - easily travels... All in all, I love the place. I will absolutely come back to CDMX, and hopefully stay at Casa RM26 again.