i use the ES as a heads up for the YM
i find that it leads SLIGHTLY the YM
thus, an entry that might be good, or a little late on ES is about right on YM, if i am watching both.
i watch both the YM and ES tape "religiously" and i see that in the tape too.
it is also sometime interesting to play off divergences.
for example, YM makes a new low, but ES doesn't.
divergences also make for nice spread trades. for example NQ was WAY WAY stinkier than YM today. sell YM @ close, buy NQ @ close as a spread example
of course triple witching wacks out all the indexes. i find it a very difficult day to trade index futures, and usually don't trade that day