There are folks who say that holster is cursed, and I've heard more than one tale of a gamer who brought a deck abusing this card to a casual gaming night and next week found their friends didn't want to play the game any more...
In all seriousness, it's worth a warning. The Legendary Holster is an incredibly powerful card that has become increasingly easy to build decks around with the expanded card pool. The ability to ace any one dude in your opponent's posse regardless of their value, bullets, or grit, is absolutely insane. As a result many people dislike playing against Legendary Holster decks, especially because there are very few effective ways to counter the card.
If playing against the Holster, you need to find ways to either avoid the dude carrying it, boot it before it can be used or lower the bullets of the dude using it to increase the chance of a failed pull. Mugging can be an effective tool for slowing a Holster deck down - just beware that if they have a Doomsday Supply in play that Holster will be coming right back if it gets discarded.
If you're looking at building a deck that utilises the Holster, I'd suggest you take a look at this article on the Pinebox forums.
If you're the type of monster who would willingly side with the Reckoners, try running the Legendary Holster out of Morgan Stables. If you don't have the advantage of the first play on a turn, use cards like La Quema and The Stakes Just Rose to move your dude with the Holster into a shootout, use the home React ability, and instantly blast an opposing dude before they have a chance to lower your bullets or play Unprepared.