I am writing this review based on my experience last month (Otc 2008) in this hotel. This was booked by my company travel agent and the rate was 96 euros (with breakfast).
I had a very bad experience in this place. Some of the main complaints

1) Rude Staff who have no professionalism in dealing with guests
2) My first room (room#109) did not even have its fan running. When I complained their solution was to open the window to make the room cooler.
3) I asked for a non-smoking room and they gave me a smoking room
4) Internet charges were 10 Euros per night but the wireless signal was very weak and my laptop would not connect. When I complained, they said that the wireless router was broken and asked me to connect using a cable. Again - No apologies or display of concern.
5) No Iron box and board in room. I am a business traveller, and what we consider as basic in US is a big deal for Holiday Inn Topkapi
6) The fridge in my room was also not working.
7) A minor complaint - The Keys would not work every night because I kept them in my wallet or shirt's pocket. It was very sensitive and had to be kept in a cover that they give you.

The only good thing was the breakfast which was excellent, checkout was fast.
So please consider if all these are factors which are important to you before booking.

In my opinion Topkapi hotel is so clean and has polite staff,but the rooms and staircase are dark. It is so easy to go to the old city and Taksim square from hotel.

I stayed at the Topkapi Hotel in Istanbul for three nights as part of an organised tour.

The hotel was being renovated from top to bottom while we were there. The rooms are small and fairly clean - I would suggest checking the cups in the bathroom before using it however as the ones in my room were not clean. The water in the shower was not a constant temperature and went from hot to cold. The shower leaked onto the floor while I was in it. The rooms do have a television (with a few English speaking channels) and air conditioning. Their is a restaurant on the top floor which we had breakfast in. There was no hot food other than tea and coffee. Breakfast consisted of bread, cheese, olives, cucumber, tomato, cerea, condimentsl and orange juice. The staff speak a limited amount of English - but are nice enough. The hotel has an elevator but no courtesy shuttle. The hotel is close to the main road and tram (five stops from the Bazaar from memory) as well as a Bureau de Change and a few restaurants. Although the hotel is in a reasonable location, I'm sure there are plenty of three star hotels in the vicinity that are just as nice if not better.