Playing Leprechaun is a lot like being at a casino. Sometimes you win a little bit, and sometimes you just screw yourself over, but the house always wins in the end. Unless you cheat by counting cards and make off with a jackpot.
First and foremost, Leprechaun is a Gold gainer, and a cheap one at that. To compensate for this, the spiteful fairy Hexes you for stealing from it. Sometimes this does absolutely nothing, like if you get Fear, and Leprechaun was the only Action you played this turn. Other times you wish you really hadn't played the card, like hitting Locusts. So what are the odds of getting a good or bad Leprechaun? Considering purely playing just Leprechaun from a five card hand, we have:
* 1 ignorable Hex (Fear)
* 3 that aren't too bad (Bad Omens, Famine, Greed)
* 4 that swing depending on what else you have in hand (Delusion, Envy, Haunting, Poverty)
* 4 that you want to avoid (Locusts, Misery, Plague, War)
So if you have a hand of Coppers and an Estate, that's 2/3 of the time that you don't really care what the Hex is. If the rest of your hand is Silvers, maybe just play those instead. But on average, you're running about a half chance of a good (or at least not that bad) outcome.
But if you're counting cards, none of this matters. Get Leprechaun down as your 7th card, and you get the pot of gold AND the rainbow, and the mischievous little cobbler even lets you get off scot-free. A Wish is a powerful thing, and if you have an engine that can reliably play Leprechaun at the right time, it is a worthy addition to your deck.
However, you have to make sure it's your 7th card. This requires careful planning - if you want to play more Actions after, you have to get in at least one village first. Prioritize sifting and draw to get to your Leprechaun in time. If he's already in hand, it's okay to get some terminals down to fulfill the 7th card condition. The main thing is to just keep track of what you're doing, and not just throw down cards without thinking.
A nice thing to look out for are Throne Room variants - while you can't normally play multiple Leprechauns in a turn for multiple Wishes, Throne Room can! Though not Royal Carriage. And maybe stay away from Prince. And Golem and Ghost.