I've had a Leon 184ps auto, with the DTUk box.
It completly transformed the car!
I did however experience some "low speed judder" in some settings, however i just changed the settings and the judder went away.
If i remember correctly i do think it was the setting for "high boost on low RPM" that caused the judder.
The box did not change the MPG to worse, however i do belive it might have improved the MPG by just a tad.
Some people bash these tuning boxes because of many reasons, one is claiming that they do not improve the actual performance of the car, which i call bullshit on..
My current car is a Leon
Cupra 290ps and before that i had a BMW M135, both fitted with a "JB4" which is also a tuning box. The Bmw M135 i spent some time with and it was a complete beast after about 1-2months of tuning and adjusting, slaughtering cars that were putting out aroun 450-500ps (320ps in my car stock..).
With my current
Cupra 290ps i have not been able to produce a custom map in the tuning box, yet..
However when spring arrives i will surely do it..
If you do not like the idea of flashing your ECU and are therefore looking to go the "tuning box"-route, go ahead!
In total ive done about 30000-40000miles with 4 differens cars, wheres 3 of them had DTUK-boxes.