Umm, the last answer was uncomplete, so i'll give the full answer.
Imposter Ditto can indeed transform as soon as he gets into battle, but he also copies all of your opponents stats ( not EV's tho ) and stat boosters eg. Swords Dance.
In other words, Imposter ditto is used mostly as a revenge killer, from a sweeper who boosted up alot of stats,
Eg. Thunderus Uses Nasty plot 3 times while Rhyperior is missing with stone edge, Thunderus uses grass knot rhyperior faints, I can send in an imposter ditto, and he will get all of Thunderus's base stats, and all boosts given to his stats, and normally people give imposter ditto choice scarf, so that he can auto matically outspeed non choiced pokemon, and go for the instant kill.
Now your opponents sweeping machine is dead, and you have your own,
Imposter ditto is also a known way to stop baton passing teams.