Gulnur's two co-joined units at the Aslan Apartments were very nice. They are not overly fancy, but are perfectly serviceable and would be a very affordable place to stay for large groups.
I do feel the need to comment on the location, though, especially after reading some of the other reviews, which in my opinion go a little overboard with praise. The apartments are located in an alley in a working class neighborhood just outside of the touristy area. Adventurous travelers shouldn't have a problem with this, but I think the average tourist might. I think that Istanbul is a very safe city in general, and it didn't feel dangerous, just not fancy at all. I think that a photo of the exterior of the building and the alley would actually be a big help in clarifying the listing.
There is a steep four-block walk uphill from the apartments to the main street where the tram runs, but if you instead turn right immediately out of the apartment and walk toward the Blue Mosque, you can get to the heart of the touristy area without climbing any hills.
I highly recommend hiring Gulnur's airport transfer, since taxi drivers will not be familiar with this address.
All in all, though, I would definitely stay here again myself.