I would say quite good, IME. I had a pair of black clowns and one day the female mysteriously died. Anyways, Ramzy gave me his small percula and initially the new fish wanted in the anenomes(I have three due to splitting) but the existing black clown was a selfish jerk and would not share even one of them. After about a month and being very persistent, the small one made it into the anemones and they are a "pair".
No mating yet, but the original black clown, which was the male has at least doubled in size and is now, I'm pretty sure, the female. The perc is still small and less than half the size of the other.