TK are establishing themselves (and IST) as a very credible long-haul operation; I've had colleagues fly to BKK, KUL on them, and come back with good reports.
That said, it feels like a stretch to fill a 319/320 every day to IST from BRS.
I remain hopeful (BD bumps not withstanding) that LH or LX will pick up a FRA or ZRH route from BRS, on at least double daily. I've been trying my hardest to use BRS-BRU as a feeder, but the range and timing of connections at BRU is dreadful, the fares are ambitious, and the lack of Saturday and Sunday flights is surprisingly limiting when trying to fly to Asia (or even the US) for the beginning of the business week.
I was pleased to see the questionnaire asking questions about the importance (to business travellers) of FFPs. Put simply, I stick with my *A FFP because it turns Economy travel (company policy) into something approaching Business (when travelling short-haul), and I can use my FFP miles to upgrade myself to Business on Long-haul. Towards the end of the CO EWR route, I was often using my FFP to put me in the Business cabin on EWR-BRS. The value of FFPs to the frequent biz traveller who has an Economy-only policy at work can't be underestimated. (And Flying Blue is not a good programme, at all, IMHO).