Ditto copies: the appearance, the stats, any stat changes made by the foe and the moves.
However a big issue with the use of Ditto is the limited PP that he suffers after he transforms. Furthermore, it would be easy for your enemy to use his powers against you-- Ditto's only choice in battle initially is to transform into its opponent. You could be transformed into something that they could then easily sweep (e.g. if Ditto is facing a Scrafty in battle, the opponent could hit you with a Fighting attack that would be both super effective against Ditto before and after its transformation.)
In short: I do not believe that Ditto is worth it. If you would like to use Transform in battle, I recommend learning it as a Smeargle (through sketch) or even as a Mew via level up. These two Pokemon are much more effective in battle, while Ditto is kind of a one-trick pony that is best used for breeding.