Depends on the game.
In Fallout games, Luck determines you crit chance and is needed for some of the better sniping perks, meaning that dedicated long range characters had to get some points in luck; AND it helped with gambling, especially in New Vegas. In Geneforge games (an indie RPG series), Luck boosts you chance to hit, chance to dodge, a number of rolls and a number of random encounters, making it a fairly useful stat; AND it's one of the cheapest stats to boost, meaning it's easy to get it high. In Elder Scrolls games, Luck boosts EVERYTHING you do, every skill. It's hard to boost it, but it's helpful.
The reason people often overlook Luck is because it tends to work behind the scenes. Most of the other Stats and Skills (or whatever they're called in your game of choice) are usually obvious, their effects openly stated. Luck frequently affects things without letting you know it did so and you just end up figuring it doesn't really do anything.
