I'm a member of an EcoBoost specific forum and there are a few folks on there that absolutely love their Flex's. I haven't really read of anyone with complaints. My wife and I are very pleased so far with her MKT, which as you appear to realize is a Flex sibling. The platform is based on the Volvo S80 which has some history on it.
Are you considering the EcoBoost or the 3.7L? I don't have any experience with the 3.7L, but it seems to be pretty similar to the 3.5L EcoBoost as well as the NA 3.5L that was in my wife's MKZ (prior to the MKT).
-Rod