For me, as a broke backpacker, hostels are my first choice of accomodation, but sometimes it's difficult to find one that has a good price and it's not uncomfortable. Well, St Christopher's inn is a great choice! I stayed in a woman 4 dorm bed, with continental breakfast included (which had expresso included guys, EXPRESSO!! How good is it ahah?). 

The hostel is very clean, with a beautiful hall, and located in the middle of everything, just take left for the castle part and right for the new town (I only walked in Edinburgh, no need to catch the buses if you're staying here). The breakfast place is also a restaurant/bar and sometimes it has parties to go (like one I saw on valentine's day).