Recently flew with TUI on a package holiday from Manchester to Zante for 9 days.
Great service onboard both flights, which were on time as well, no waiting for luggage when we returned home either. Nice hotel with helpful TUI rep and the app was very informative, before, during and after our holiday.
The only issue I had with the app was prior to the holiday, I was not able to enter our parties passport information, I had to log onto their website, not a big problem, but it would be more convenient to allow passengers to use the app for all functions.
Overall, I am seeing an improvement with TUI...I'm quick to complain when something goes wrong, but I am also willing to share positive experiences, which seem to be few and far between in the world of social media etc.