Rulings

• Each other creature copies the printed values of the targeted creature, plus any copy effects that have been applied to that creature. They won’t copy other effects that have changed the targeted creature’s power, toughness, types, color, or so on. They also won’t copy counters on the targeted creature (they’ll each just retain the counters they already have).2008-05-01
• If the targeted creature is itself copying a creature, each other creature will become whatever it’s copying, as modified by that copy effect. For example, if you target a Cemetery Puca that’s copying a Grizzly Bears (a 2/2 creature with no abilities), each other creature will become a Grizzly Bears with the Cemetery Puca ability.2008-05-01
• This effect can cause each other creature to stop being a creature. For example, if you target an animated Mutavault (a land with an activated ability that turns it into a creature), only the printed wording will be copied — the “becomes a creature” effect won’t. Each other creature will become an unanimated Mutavault.2008-05-01
• Noncopy effects that have already applied to the other creatures will continue to apply to them. For example, if Giant Growth had given one of them +3/+3 earlier in the turn, then Mirrorweave made it a copy of Grizzly Bears, it will be a 5/5 Grizzly Bears.2008-05-01
• If the targeted creature is a token, each other creature copies the original characteristics of that token as stated by the effect that put it onto the battlefield. Those creatures don’t become tokens.2008-05-01
• As the turn ends, the other creatures revert to what they were before. If two Mirrorweaves are cast on the same turn, they’ll both wear off at the same time.2008-05-01