We loved our stay at this hotel. It is very close to everything a tourist might like to see in Istanbul. The breakfast was excellent, and the staff was most helpful.
However, if you are a most robust person, you might want to search for a hotel with larger rooms and bathrooms. Also, some repairs/refurbishing wouldn't hurt.

I booked at a very good rate and didn't know what to expect. The hotel is in a very pleasant quiet side street, surrounded by other small hotels, a couple of minutes from Gulhane tram stop, and 5 minutes from Topkapi Palace, 10 minutes from Hagia Sofia.
I had a double room for single occupancy. The shower had seen better days so I asked for a change of room and got it with no problems.
The breakfast was very basic, but adequate, although by the end of a week I was glad of a change. The free wifi worked well.
The staff are pleasant and helpful, and speak very good English.

I would use this hotel again if I return to Istanbul.

Ersu hotel was recommended to me by a friend who had stayed at the same hotel. But when I checked-in the lobby had a rancid smell along with the smell of smoke (unavoidable I guess in Turkey). The room was small but good enough for a single person. the room looked clean but the bathroom was the dirtiest I have seen....the shower-head had all sorts of gunk on it and the water flowed out of the stall in trickle. The food was substandard....they had the same fare throughout my stay of 4 days.... there was hardly any fruit or yogurt served during breakfast. THis was a disappointment. I guess you get what one pays for and the room rates are quite cheap compared to other hotels in Sultanahmet.

A bit dark, a bit noisy, dated basic rooms and not very clean; bathroom ok and clean; staff very friendly and willing to help, location - PERFECT. For the price paid (30-40 EUR per night a standard double), it was a fair choice and i would stay there again.