She looks down at you giggling.  You look down to see a pool of cum but you're confused.  She took it from you but yet you felt very little.  You felt the excitement of an orgasm brewing but then no immense pleasure....no high.  The look of satisfaction on her face tells you it wasn't by accident, it was intended that you get no pleasure from this orgasm.  And that in essence is a ruined orgasm.

Put simply a ruined orgasm is when you are brought to the brink of orgasm, the ejaculation response triggers, but all stimulation is ceased before the pleasure response can trigger. This knowledge in the hands of someone well practiced can be exercised in the most cruel and sadistic way. You see, when your orgasm is ruined, yes you cum, but no you don't get anywhere near the full enjoyment of it and worse yet you remain in a state of desire afterwards. So unlike when you typically cum and have a come down afterwards and lose interest in sex albeit briefly, this keeps you in a state of constant want.

So now lets layer in chastity. You have spend the last week, month....longer perhaps in your cage and you think you have finally earned an orgasm. However the dominant in the relationship has other ideas. They want to keep you subservient and wanting, so when they tie you down, you both are thinking about a release, just in very different ways! 

This in part is one of the many beauties of chastity and orgasm control, some dominants will use ruined orgasms as a punishment for an indiscretion whilst some will enforce a 'all orgasms must be ruined orgasms' sort of system. Ultimately it's in their hands, heck, you might just get off on knowing you're never going to full get off again...ironic huh? 

For the dominant it's a tool to control your mind.  Knowing that they can give you a release when they wish but whilst being able to keep pleasure at bay and therefore constantly on your mind.  That keeps the key holder firmly in the mind of the chaste at all times, always wanting.  So in answer to the original question of is it heaven or is it hell...well, it's both.