I had a pleasant time at Astor Hyde, the building itself is very impressive and the staff was helpful. Mind you, the 3rd and 4th floors are definitely a hassle if you have mobility problems; no elevators and very high stairs may be a problem to you if you can't move about easily. Other than that, I had a nice time. A bit on the formal side though... but cool! About a 10 minute walk to the nearest tube station.
Definitely a recommended Hostel. Had no issues while I was there, the staff was very friendly and the atmosphere is perfect for you to meet everyone else; they even set up a free dinner with everyone who was there one night, very cool! Location is also great, in 5 minutes you can walk to the historical part of Istanbul and within 1 minute you're near the Tram station. Would go back without looking for another place!
Couldn't ask more of a hostel really. The people were very friendly, I've always felt pretty safe and the atmosphere is really rather good, I've never felt like I was just another head there. Highly recommended, would stay there again!