This is a fun card. It's unusual in that it's both a counter and an assist to decks that it's naturally made for playing in. If you make this thing active, then Rare Candy is out of the question, both for you and your opponent. This can be bad for an opponent playing Machamp, Cursegar, or anything depending on Rare Candy to get a Stage 2 into play quickly. In it's own way, Spiritomb acts as a necessary replacement for Rare Candy. In one turn, you can evolve a Basic Pokemon into a Stage 1, then play the Stage 2 from your hand on the next turn.
There are many other uses for Spiritomb. My favorite (until rotation) is to use a Roseanne's Research to get both Spiritomb and Baltoy in play, then make Spiritomb active and use Darkness Grace to evolve your Baltoy to Claydol. With this simple trick, you can gain some fast early-game momentum. Come rotation, Pokemon Collector will be an appropriate replacement for Roseanne's Research, and you can try either the Ninetales or Dodrio line to replace Claydol. In fact, this trick is great for getting a lot of evolved pokemon into play quickly.
SP decks won't like this card, until Dialga G lv X comes into play. While Spiritomb is active, the SP player can't use PokeTurn, Energy Gain, or most of the other cards that they love using to turn the game around, all while Spiritomb's player is building a stable of highly-evolved, dangerous pokemon. Spiritomb can't slow SP decks down once Dialga G lv X is in play, though Spiritomb would make it a little difficult for them to fish up the lv X card to bring into play. SP pokemon will want to knock this card out soon, which may not be difficult to do with only 60 HP, and Spiritomb doing nothing to damage them. A good solution may be Blaziken FB, which will make something else active and burn it, with the bonus of a burned pokemon not being able to use it's PokePower.
If your deck relies heavily on evolved pokemon, Spiritomb is almost priceless, though if you don't have any, it should stand to reason that this card won't do much for you. Early in the game, this card is great for setting up, but later in the game, it won't be as useful as the cards it sets up.
Spiritomb from Platinum Arceus gets a score of 4 out of 5.
It's a useful card that supports and disrupts at the same time. Just don't expect it to be played for it's damage output, as it practically has none. If you play the Pokemon TCG competitively, you will see this card. A lot.