Turkish business class is generally quite good. The big negative is most long haul planes still have an older product which is almost lie flat with no direct aisle access for all seats. As a plus the short haul from FCO-IST will have recliner style 1st class seats vs the blocked middle you see in most Euro Business flights.
Catering is really good and bedding is nice even though you don't get a true flat bed (it's close enough for me to sleep from ORD-IST).
I have not been to the lounge in the new airport but they are generally well regarded for their IST lounge, although as noted the lounge can be crowded at times.

If you can't get a direct business class flight and want business they are a solid option compared to other *A European carriers. I generally don't look at business class ex Europe though as it's a daytime fligtht so I don't need a full recliner, economy plus does me just fine.