I've used Dykem in the past, and it appeals to me. Thinking the main appeals would be....
? It would color the entire rear of the case, vs. just a small slash mark. Therefore easier to spot your own at a match.
? It would fill all the crevices around the primer so there's no way it wouldn't stay marked.
? Dyken comes in numerous colors. I might choose pink or green as another differentiator. (Of course green doesn't work if your match areas are grassy. If you shoot near San Francisco, someone is probably already using pink.
)
Downsides might be...
? Maybe not quite as economical application wise, by using the cotton ball applicator. (I assume you're not talking about spray.)
? Might turn your plastic ammo boxes into a mess after 10 applications, with all the Dykem drips and splatters.
? If Dykem did drip onto the plastic ammo box, how would the plastic respond?
? I stopped using it because my 4oz can dried up. Using up an entire 32oz can might take some doing. Your best bet might be refilling the small can from the big. That limits the number of times the big can is opened.