This flex cable is great for locking up other parts of your bike. I use it in addition to my U-lock.
While this cable alone is poor security, using it in addition to a u-lock allows you to lock up other parts of your bike.
I have 2 and use one threaded through my Wald quick release basket when I leave it on my bike.
I use the other flex cable to lock up the front wheel as I put my U-lock through my rear tire and frame.