I use one with my Hayward Tiger Shark. One thing I would add is make sure to use at least at 12 gauge cord. 10 Gauge would be better. The lower the gauge, the thicker the cord and the less voltage drop you will have. And get one that is only as long as you need it. The shorter the better. Again, less drop in power.
I found the service manual for my robot on line. Many of the trouble shooting issues involve not enough voltage getting to the unit due to the use of a cheap, thin extension cord that wasn't supplying enough voltage.
I used an ext chord on a shop vac while sucking up leaves around the pool. The vac accidentally rolled into the pool... blew the ground fault device instantly. I let the vac dry out and still functions! Don't think it's be swimming with a robot attached to an ext chord though.