I can't ever recall seeing him play but it's safe to assume he's at least twice as good as El Ghazi or Hassan. So without knowing anything about him this is a huge upgrade.
That's what worries me. When fans of the club who he plays for are telling you he's shit then he's not a player you should be signing. It's when fans are unhappy to lose a player that you know he's good.
I am sure if you make the effort Istanbul wouldnt be the worst place to live but you hear so many stories of players like Darius vassell just living in a hotel for months. For someone like Lennon I dont think it's a good move.
Not a smart move for someone who has issues with depression/mental health. It always amazes me when already rich footballers make the move to Turkey. Hope it works out for him.