Opinion on some Istanbul hostels

I have stayed at the Cordial and really liked it. There might be nicer hostels, but the staff was really nice and helpfull. Being a single female traveller some of the staff invited me to go a night into town with them, so I would not have to walk around on my own and they did a good job looking after me (without hitting on me or having intentions, btw).
The only thing that spoiled it a bit was, that they overcharged me and I only noticed once I was back home. I did communicate with them about it but they made no effort to reimburse. It wasn't an awfull lot, but still.

When I was there they were working on a nice roof-top cafe/terace, maybe that is finished now.

I stayed at Cordial House a couple months ago. It's comfortable and the location near the tram stop is excellent. Be aware, though, that they do charge a currency conversion fee if you pay in Turkish Lira and not their preferred Euros. Oh, and they were the only Turkish hostel that didn't honor my reservation request for a lower rather than an upper bunkbed. If the currently quoted prices are similar, I'd suggest you look into booking at the Antique Hostel. It's also comfortable, the location is pretty good, and the basic rate includes breakfast and internet.

You're wise to book ahead for Istanbul. If you want a particular hostel, shop more than one booking site. I found that what shows as booked up at one is often available at another. Besides hostels.com, another good one to try is hostelbookers.com.