Just completed a roundtrip on one of the cheap G fares - ORD-IST-ORD.
Thought I would provide some detail around the experience for others...
Check in is at Terminal 5 @ ORD. There is a separate line for business class check-in which includes TK E/E+ and *A gold. Agent will issue a separate lounge invite which specifically states "Business class lounge" on the top. Interestingly enough, there is a separate business class line for security.. the TSA person saw the lounge invite and waved us over to the biz security line, even though we were in coach.
YMMV..!
TK now uses what used to be the JAL Sakura lounge by M-7 gate. Food offerings are scant, decent beverage selection. Can get crowded but there are other *A lounges, though nowhere near the departure gate.
Boarding was straight-forward.. TK E/E+ were allowed to "board at their leisure", even before families with small children and elderly.
The A330 is decent, legroom okay (may be blocked by electronics box), footrest. Seat reclines and slides forward at the same time. Small amenity kit containing eyeshade, socks, lip balm, toothbrush/paste, and earplugs is passed out after take-off. Very nice, even for economy passengers! Actual amenity kit container varies -- outbound had a small red pencil case bag; inbound had a metal box for children. TK catering is ample with lots of beverage service -- very difficult to go hungry on these flights unless you are extremely picky. AVOD is okay -- expansive movie and audio selection. Earbud jack is dual-pronged so bring a converter if you want to use your own headphones.
There was also AC and USB power underneath the seat but only the USB was working for mine.
*Note: families with babies are likely to be in bulkhead row.
Arrival was easy -- there is a priority immigration line (after you get your visa sticker) for business and TK E/E+. It's past the entrances for Turkish and foreigner lines (rightmost line if you are facing the immigration desks).
No arrival form, stamp stamp, and you are done.
Return was a little confusing. The check-in area for our flight was in the "B" area but the security agent wanted a print-out of the itinerary. Because this is a US-bound flight, expect several rounds of security questioning before check-in and at the gate area. There is a dedicated business class / elite check-in area but it is "F" area, I think. TK lounge is amazing, of course. The screens will say "GO TO GATE" very early.. this just means that you need to be aware of the extra security questioning before entering the gate area. Sometimes there is a line of people here.
Boarding for the return flight was very odd. There weren't any announcements made in the boarding area.. there are monitors that flash boarding announcements quietly (no noise).. be careful! Return flight was fine, same as outbound.
Miles posted to TK M&S about 2 days after flight - very fast!
Overall a positive experience.^