I'd say it is usually worth keeping it in your starting hand if you have a solid curve of minions already(this goes for evolve shaman, and aggro/tempo decks). If you are running it in a more reactive deck(yes, people actually do that, the card is broken to that extent) you can keep it alongside an early game board clear like lightning storm.
Remember that the card needs to have minions die in order to be playable, so you need to either clear a large board with an aoe effect or do some trades with your own minions.
Outside of this, you can keep the card in basically any MU, as your opponent will likely either be playing his own swarm of minions, or removing your own in the early game, so minions will die either way.
Corridor Creeper is a great keep in aggro mirrors, I keep it while I play tempo rogue against paladins and other rogues, and it is not so bad against control decks, as they have early board clears almost always. Against control, only keep it if you already have a good early game though.
You should almost always keep it, very rarely it isn't worth it, even if the rest of your hand sucks just mulligan the other cards, only in very specific match-ups were you absolutely need a specific card or you 100% lose, the creepers get worse as the game goes on, if you top-deck a creeper late game it can be really bad for you so keeping 1 gives you an insane tempo play fairly early and also makes your later top decks better.
I personally don't think you should keep creeper in your starting hand if the rest of your hand isn't good, unless the deck you are playing has a very low curve and doesn't run too many spells(ie face hunter, aggro druid). I certainly wouldn't keep it in something like tempo mage for instance.
If you have anything even approximating a decent early game curve, or an early game AoE, I'd keep it. Unless your opening hand is something like Corridor Creeper, Bonemare, Bonemare, I'd keep the creeper.