Have used several times on my training rides. I make my own GU. Issue for me is that it is not easy to use one-handed. Taking two hands presents some additional risks while cycling. Furthermore a few times the entire top rotated, vs just the spout. Thus I will continue to use on training rides but in other rides will likely continue to use the packets that I can open with my teeth.
Other than the hard-to-loosen nozzle, the other serious downside is if you don't work thru the batch in your flask relatively fast, then the gel hardens in the flask corners and along the bottom edge. Short window to work through 5 packs' worth. So back to the individual eco-insensitive packs I go ,)