Mine repeats well but it fades too quickly. It's only yellow for a day.
Carefree Sunshine, Radler's first yellow, holds its color better but the flowers are smaller.

I have heard other people complain about it too. It's not as vigorous a bloomer and the blooms are messy. Good yellows that don't fade are had to find. The best one I have is definitely Julia Child. She does fade some but not to white and she blooms her fool head off!

I bought my Sunny Knockout from our local garden center in a 3 gallon container and it's been in the ground about a year and a half. It blooms continuously, doesn't get sprayed and has never had one speck of disease. The bush itself is a slow grower (cut back to about 8 in tall after our last freeze in february, and is now a little better than two feet tall and wide). To be fair on that count, the bush regularly gets abused by my dog, who tends to barrel straight through it to get to the back fence. The blooms DO fade very fast. It's planted in an area that doesn't get very much attention, so very little extra water (and we've not had much rain). It's definitely not an exciting rose, and when I re-do that area in the fall, I'll most likely replace it with something else. Or maybe not. I can't really decide. Not having disease problems makes it VERY alluring. I don't love it, but I don't dislike it. Sigh.

I don't care if it's clean, it is one sad excuse for an introduction. It fades to all white so quickly one can hardly declare it to be sunny.

I agree with amandahugg, I looked at the pinks at HD and they were a sad excuse for pinks, and thought what is all the raving about for knockouts, then a few days later, the yellows came in, it looked glorious with all its fresh blooms, bought it on impulse, have had it for a week now, all the blooms are washed out almost white. If they were true white, it would be ok. but really washed out looking is not OK. Are knockout roses really only for hot dry climates?

I agree that Carefree Sunshine is much better.

Also agree that Carefree Sunshine is a much better rose.

Another to consider, that is more of soft lighter yellow is Solero Vigorosa.

Here is a link that might be useful: Solero/ Solero Vigorosa

Julia Child and Sunsprite . Very little fade. Ht form, Henry Fonda.

Consider Sunflare, one of the best recent yellow introductions. It does fade some, but the clusters are very pretty in varying shades of yellow. Rebloom is excellent and the shiny green foliage is most attractive.

Here is a link that might be useful: Sunflare at HMF