Ok, i hear a lot of people dissing on ditto, one of the best cards.  And I'm backing up wheterman.  He stated "If you face a deck with sucky cards, Ditto becomes those sucky cards."  Yes, but your opponent also has sucky those sucky cards(just adding on to what you said about that).  And any sane person would play more pokemon than just Ditto.  Like I play him in an "Anti-Haymaker/Haymaker", with Chan, Mewtwo, Ditto and Scyther.  Ditto works wonders.  But in any tournament deck, you won't be seeing to many Muks and dumb cards like Weedle.  A good deck with Ditto can get around these little "speed bumps".
Muk - Gust and Kill
Sucky Pokemon - Retreat for good pokemon
Mime - That is why WotC blessed us with other pokemon besides Ditto that do less than 30 damage a turn(Jiggly, Chan and Lapras)
Rattata - Please, if you are facing a Rattata and are having trouble, kick yourself in the pants and buy a theme deck
Sleep/Confuse/etc... - Scoop Up, Switch, Digger(Get those last 10 HP off Ditto, and since you will probly keep flipping tails because you need Ditto dead, then you just KO'd a pokemon[I was joking about the whole Digger thing, and if you are still reading STOP])
Remember, there is always a little trouble whenever you play any deck.  Ditto is just something you have to deal with to get AWESOME POWER.
 
And I know I pretty much repeated whatever wheather man said but oh well.  I had fun typing and wasting 7 minutes of my life typing this.
 
Daniel P.
neopolatin@earthlink.net