I flew TK IST-CMB (via MLE) on a 333 in J last week, and have flown them on this route several times previously.
I find the TK long-haul product to be generally OK, but not outstanding. The seating configuration of 2-2-2 isn't class leading in any sense, but the lie-flat seats are quite comfortable and easily accommodate my 5' 10" frame. Service tends to be OK too, but again nothing special. Same for the food. TK is not my Star Alliance airline of choice for long-haul by any means, but they're certainly not the worst.
OTOH, TK is my preferred airline for intra-Europe flights. Their business class on many of the A320/1's and 737's is excellent, with individual recliner seats in a 2-2 layout (though on some flights you do get the more usual Euro layout of 3-3 with the empty middle). I had a great flight a couple of weeks ago from IST to FCO on a 321, where the service, catering and comfort were top notch (even had a "chef" in the galley on board).
As for the lounge in Istanbul, it's huge and is fairly comfortable, with reasonable food/drink options and general facilities. The main problem I have with it is that it does get very crowded at times, where you may have to fight for a seat.
Hope that helps a bit. Would be happy to expand on any of the points.