[thejeff]

Well, in the classic Master Enslave contest, he who goes second has some serious issues to contend with.

First, the defender can cast AntiMagic/Army of Lead to protect both his troops and your former troops, greatly weakening the effect of your Master Enslave.

Second, since his new troops are among your remaining troops (and mages) many of your mages will be attacked or in hand-to-hand and thus less likely to cast what was ordered.
Do enslaved troops attack the round they're enslaved? I think they do.

This applies less if it takes a round or two to get communioned up to Master Enslave level. Though even a communion can do it round 1 with a Matrix and a late acting caster.