Clefable is a useful card that seems to be rather underrated.  Metronome
allows you to do your opponent's best attack for one energy.  You also don't
have to worry about discarding energy.  This makes Clefable especially great
against fire Pokemon, as many of them require discarding.  Her second attack,
Minimize, is pretty good too.  If you are trying to power someone up on your
bench, use Minimize to keep Clefable alive until you are ready to retreat. 
Like most Colorless Pokemon, Clefable is resistant to Psychic.  Since cards
like Movie Promo Mewtwo are played frequently, the resistance is a plus.
Clefable has some downsides as well.  First, to get her you have to play
Clefairy, whose 40 HP makes her an easy target.  Second, a Fighting weakness
is a problem because of Hitmonchan.  Finally, 70 HP can sometimes be a
problem.
All in all, I feel Clefable is worth playing.  You might want to include
Fighting resistant Pokemon to combat her weakness.  I would also recommend
not putting Clefairy on bench without a Clefable in your hand if you can help
it.

On a side note, I received a plethora of e-mails about my Imp. Prof. Oak
article pointing out that I incorrently stated that your opponent discards is
or her hand.  He or she in fact shuffles it into his or her deck.  I
apologize if I confused anyone.  I guess I should read the cards a little
more carefully.  If anyone wants to e-mail me, my address is Woof246@aol.com
~Madame Butterfly