The ghost is the Anti-hero....hero, not ment ot be bought for aggresive hero attacks, the Ghost is the counter to any of those pesky upgraded heroes that keep attacking you. With its ability to cloak and use its "Snipe" ability -- which does 1500 damage to the target hero, the Ghost is truely the counter to almost any hero. Along with its EMP which is highly effective against Stalkers, Void Rays, or Zealot mercs from other players, an EMP will also completely deplete an Archon's shield or Queenfestor's mana, so its not a bad idea to purchase a ghost solely to counter these two heroes.
The snipe for the Ghost can be upgraded to do even more damage to heros even making the Archon seem like nothing.
The Ghost can be upgraded to a Spectre which has the ability (for a price) to nuke and is always cloaked. Along with high damage and its Mind Lash ability -- which does 4000 damage to the target hero, the Spectre is truely the ultimate Anti-Hero unit. Like the quotation in the tooltip says, getting a spectre will make you hit a hero so hard, no one will want to buy another hero again. The Spectre is a hero best used only when going up against an Archon or a very powerful Colossus, and can even be used as a minor unit/merc harass unit, since the Spectre has the longest range in the map, and have a fast movement speed.
The Former Spectre (Version 0.9.9) was a scaled up Ghost model, due to lack of Spectre models during the Starcraft 2 beta. Since the release of Version 1.0, the Spectre's model has been replaced with the true Spectre Model, and the "Spectre Snipe" ability was replaced with "Hero Mind Lash".