9 / 10
Gardevoir has an amazing Poke-Power. Using two supporters per turn is game changing and allows decks using her to have an amazing degree of consistency. Gardevoir's attack is also very nice. 60 for 3 isn't that special, but consider that it's PCC, meaning she's more splashable than some other recent DP Pokemon, and that the attack shuts off powers and it really starts to shine against some decks. Gardevoir's type, Psychic, is very good and is strong against many other Pokemon.
Combo wise Gardevoir has a nice level X, a parallel evolution in Gallade, and two other sister forms in Modified, Gardevoir PK and Gardevoir ex d from DF.
The downsides to Gardevoir are the fact that she is a stage 2, has relatively low HP, Psychic weakness, and a retreat cost of 2. These can be worked around but need to be considered when building a deck using her.