Just returned from an IAD - FCO trip via Istanbul in business.
This was my second time on TK in business and I like it a lot. I don't think it is on par with the top Asian carriers, but it beats United & Air Canada easily. Food service is far superior and, subjectively speaking, the flat bed seems easier to me for sleep. The cabin crews seem uniformly engaged at a high level and are friendly & responsive. This was on the A330. Something to note - the plane on my outbound long haul had free wifi, the return flight to the USA had no wifi.
The IST - FCO - IST segments were B737 & A321 respectively. As noted by others business on these planes consists of a blocked middle seat with a tray like device. Even on these short hauls food service was plentiful.
Three of my four arrivals & departures from IST involved getting bussed to the terminal, which isn't the most convenient way. Hopefully they will do better in the future. For those crediting mile to UA, there is a likely loss of points bonuses. In my opinion it's worth the loss if the TK fare is attractive. Besides, you will have a happier experience on TK.
Last, I really do like the lounge at IST.