Stayed in GBT Hostel in August 2010 during my Trans-Siberian journey. The place is located in a very small residential street and was quite difficult to find at first. It´s a long-long walk there from the train station - at least it definetely seemed like that at 4 in the morning with all the backpacks.
It´s a small house with just few rooms. We stayed in 4-persons´ room. There´re 2 single and one "double" bed in that room. The double bed is just few boards nailed together with the mattress over them. The mattress was well-worn and hard, the blankets quite filty. Luckily the bed-linen was clean. The sleep quality was definetely not of my best - actually it was not relaxing at all.
The other issue was the cleanliness of the place. Although the family tried to keep the place clean there was quite dirty batrhoom, no toilet paper (no problem, just bring you own!) etc. I guess the main problem were the people staying there that time - they didn´t even cleaned their dishes after themselves.
On the bright side, there was free internet access and computer for use. The place fitted Ulan-Ude and Trans-Siberian trip perferctly. The family was helpful and I was able to use their phone to arrange my bus tickets to Ulaanbaatar. So in general it was quite nice experience - something you may not enjoy but you still like it. If ever again in Ulan Ude I´d stay there again. But they really need to signpost their place far more better.