I stayed at Hostal Arias
for two nights last month. The location is very good, in a quient street very close to the major sights. The room itself was clean and comfortable. The only negative thing I have to say is that when I tried to check out at 11 pm the night before I left I was told that was not possible and that a cashier is only available from 10-6 every day. Being that I had to leave very early the next morning I left a note in my room asking them to charge my credit card for another night....only to find out upon leaving the next morning that the new desk person was going to take a payment from me.
All in all, a very decent place to stay, but make sure you speak a decent spanish because most of the staff there does not speak English.