As soon as the user comes into battle, it transforms into its opponent. Imposter copies the target's appearance, cry, moves (which all have 5 PP), Ability, stats (except HP), stat changes (but not stat multipliers such as with Choice Specs), weight, and Mega Evolution or form (regardless of held item).
When Ditto transforms, it copies its opponents exact stats. All of Ditto's stats after transformation are irrelevant, except for HP. Yours has a perfect HP IV. I say it's worth keeping if you're using it for battling. For breeding, however, 5IV-6IV Dittos are much better.