It is quite reasonable accomodation in Yerevan as it offers room at half/one-third price compared to other hotels in Yerevan. For the money, you get what you pay. Quite a good accomodation, which is rather basic and you can "touch" the atmosphere of the Soviet era here ;). However, rooms were clean and equipment of the rooms was sufficient enough for a good stay. I stayed here 2 (june 2013) and 2 (july 2013) nights and would come back again.
The YSU Guesthouse is at walking distance to major Yerevan attractions (e.g., Cascade - 3 min, Republic Square - 10 min).
If you want to send postcards, post office is just behind the corner (1 min walk). On the opposite side of the street is the store selling the Armenian traditional cognacs.
If you stay here, be prepared to that the basic knowledge of Russian helps considerably in communication with the staff. English really doesn't help much here.