Same here. I am impressed and pleased with the ease of use the Plug-It system gives me. I also plan on fitting all my other tools with this thanks to the small extension you can buy and fit to non Plug-It tools.
I never had the problem of it coming loose from my Rotex. In fact, the first time I got the machine I had a very hard time to take it off. And it wasn't that easy to put it on again too. This got better over time, like people said above.
Nevertheless, I think the OP has a point when he says it is poorly designed. It feels a bit flimsy to me too, the plastic feels too weak and I am worried that if I use it too much the connector might break. I do hope Festool will improve this design in the future with stronger plastic, a stronger connection and more of a snap-lock feel to it than what it has now.
Btw OP, you shouldn't pull too much on the cord, that is never a good thing. If you feel you need a bigger range you can get a longer 7,5 meter cord instead of the 4 meter standard cord.